A REPORT ON
AUTOMOTIVE FUEL ECONOMY
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U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR AND WATER PROGRAMS
MOBILE SOURCE AIR POLLUTION CONTROL
OCTOBER 1973
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A REPORT ON
AUTOMOBILE FUEL ECONOMY
October 1973
United States
Environmental Protection Agency
Office of Air and Water Programs
Office of Mobile Source Air Pollution Control
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CONTENTS
Page
I. Introduction 1
II. Summary and Conclusions 2
III. Data Sources and Calculation Procedures 6
IV. General Factors Affecting Automobile Fuel Economy 8
A. Engine/Vehicle Design 8
B. Vehicle Operation and Use 25
V. Trends in Automobile Fuel Economy 31
VI. Appendices 36
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I. INTRODUCTION
This report is the second of two EFA reports on automobile fuel
economy. The first report, entitled "Fuel Economy and Emission Control,"
was published in November of 1972, and dealt with the subject of automobile
fuel economy primarily as it was affected by vehicle weight, convenience
devices, and emission controls. This report is a sequel to the earlier
report in that it contains discussion of automobile fuel economy, but it
differs in some respects, reflecting the results of further study. The
data base has been expanded, the calculation procedures have been refined,
certain areas have been reexamined using newer data and/or different analysis
techniques, and additional vehicle design and operating parameters that
affect fuel economy are discussed.
Since the earlier report was published, interest in the subject of
automobile fuel economy has increased greatly. The earlier report was the
subject of comment from within EFA, other Federal agencies, the Congress,
state and local governments, citizens, fleet purchasers, motor vehicle
manufacturers, and fuel producers. This report is intended to be of use
to these same groups. While this report does not discuss the detailed
technical analyses and background from which much of the data were derived,
it does provide sufficient information upon which to make informed decisions
regarding the purchase and operation of an automobile, and from which an
understanding can be had of the most.important parameters affecting
automobile fuel economy.
For those seeking a more technical and detailed presentation of the
topics discussed here, additional information can be obtained from the
references listed in the bibliography at the end of this report.
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II. SUMMARY AMD CONCLUSIONS
A., SUMMARY
The Environmental Protection Agency has analyzed fuel economy data
from more than 4,000 cars (of.which over 1500 were equipped with emissions
controls);tested on the,Federal Driving Cycle. A carbon balance equation
was used to calculate fuel economy. Statistical regression techniques
were used to determine the effect of various design parameters on fuel
economy.,
The data were derived from EPA certification, surveillance and in-house
evaluation testing. This is the most extensive data analysis known to have
been performed'on this subject to date. It is also considered by EPA to be
the most accurate for the purpose of comparing changes in vehicle fuel economy
because of the use of a single consistent driving cycle and controlled ambient
conditions.
In addition, the EPA has evaluated a significant amount of new data
which have recently become available (see bibliography) as well as older data
which have recently come to light. Much of this data was generated by automobile
manufacturers. * Significant data were also developed by the U.S. Department
of Transportation. Important information also came from a study on vehicle
weight trends which was performed under contract to the EPA. Much of this
additional data concentrated on the Impact of changes in vehicle design
and vehicle operation on fuel economy.
This study indicates that vehicle weight is the single most important
vehicle design parameter affecting fuel economy. Fast and future increases
in vehicle weight have had, and will continue to have, a significant adverse
effect on fuel usage. Weight is a parameter over which the car buyer has
direct discretionary control, in terms of the size car he choses to purchase..
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Other aspects of vehicle design (size, tires, axle ratio, engine
compression ratio, air conditioning, transmission type, emission controls,
and engine size and type) and operation (speed, trip length, acceleration,
maintenance, road surface and grade, and elevation) were also examined.
Changes in individual vehicle design parameters, including weight, are shown
to affect fuel economy from - 50% to over 4- 100% of the nationwide average
fuel economy. The most important of the operating parameters can individually
vary the fuel economy of a given weight vehicle over a - 60% to + 25% range.
The sales weighted average fuel economy loss due to emission
controls for 1973 vehicles compared to uncontrolled vehicles is 10.1%. This
penalty, while significant, must be viewed in the context of the other penalties
being experienced by today's car buyer. These include penalties of 9%
to 20% for air conditioning and 2% to 15% for automatic transmissions. The
loss due to emission controls has varied significantly with vehicle weight,
with lighter cars showing a gain of about 3% and heavy vehicles suffering
losses up to 18%. Despite the many statements regarding the loss in fuel
economy due to meeting the 1975/1976 standards, it now appears that vehicles
equipped with catalytic converters to meet the 1975 standards will have
improved (0% to 15%) fuel economy over 1973 vehicles.
The use of engines other than the present spark-ignition,
reciprocating engine could have a significant impact on vehicle fuel economy.
Use of the spark-ignition, rotary engine presently results in significant
losses in fuel economy, while the diesel engine offers a significant increase
in fuel economy.
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Clear trends have developed over the past ten years in motor vehicle
fuel economy and factors affecting fuel economy. Vehicle weight has been
increasing since 1962 for individual models and the population as a whole.
This steadily increasing average weight trend has been accompanied by a
steadily decreasing average fuel economy. The use of emission controls
has had little impact on this trend. Whether the increasing market share
of smaller vehicles will have a noticeable effect is yet to be seen.
B. CONCLUSIONS
1. Vehicle weight is the single most important parameter affecting urban
fuel economy; a 5,000 pound vehicle demonstrates 50% lower fuel economy
than a 2,500 pound vehicle.
2. Vehicle weight, for both individual models and the sales weighted average,
has increased significantly from 1962 to 1973 and current trends indicate
additional increases in the future. This weight increase has accounted for
about one half of the total drop in the average fuel economy of these model
year vehicles.
3. The sales weighted average fuel economy loss due to emission controls
(including reduction in compression ratio) for 1973 vehicles, compared to
uncontrolled (pre-1968) vehicles, is 10.1%. However, vehicles less than
3,500 pounds show an average 3% gain (attributable to carburetor changes
made to control emissions) while vehicles heavier than 3,500 pounds show
losses up to 18%. The size of these losses, however, is highly dependent
on the type of control systems the manufacturer has chosen to use.
4. Prototype conventional engine powered vehicles equipped with catalytic
converters designed to meet the statutory HC and CO standards are expected
to show fuel economy improvement over 1973 vehicles up to,12%.
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5. The fuel economy penalty due to the use of convenience devices such as
air conditioning (a/c) or automatic transmission (a/t) is comparable to that
due to emission controls, and can range from 9% to 20% for a/c and 2% to 15%
for a/t.
6. The reduction in compression ratio employed by most manufacturers to
enable their vehicles to operate on 91 octane gasoline has resulted in a
3.5% fuel economy loss. However, a large share of the cost penalty due to
that loss can be regained by using the (presently) less expensive 91 octane
fuel for which the engine was designed.
7. The way in which a vehicle is operated significantly affects vehicle
fuel economy. Among the most important parameters, high vehicle speeds
and short trips can have an adverse effect on fuel economy of up to 60%.
8. Future trends, including increased vehicle weight and possible use
of the rotary engine, may result in significant (20%-35%) fuel economy
penalties.
9. The diesel and open chamber stratified charge engines show better fuel
economy than the conventional engine with the diesel showing a fuel economy
improvement of more than 70%.
10. Today's car buyer has available a choice of vehicles in terms of the
size and weight, engine type, and convenience devices. These choices can
influence a vehiclefs fuel economy over a range of 4 to 1.
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III. DATA SOURCES AND CALCULATION PROCEDURES
The data used to derive the fuel economy information for this report
originate primarily from EPA Certification and Surveillance programs, as a
byproduct of the emission tests run to determine compliance of new motor
vehicles with the emission standards and to determine emissions from in-use
vehicles. Other data originate from in-house EPA testing of exhaust
emission control retrofit devices and advanced prototype vehicles, contracts
funded by EPA, statistics from the Department of Transportation,* the existing
literature, and information submitted to EPA by automobile manufacturers.
The fuel economy data derived from the emission tests are obtained by
the carbon balance method. Basically this involves taking the unburned
hydrocarbon (H£), carbon monoxide (CO), and carbon dioxide (CJ^) emissions
from the emission test and calculating the fuel consumption for the test,
using the fact that the HC, CO, and C02 represent all carbon containing
constituents of the exhaust, and the fact that the fuel itself consists
of hydrocarbon compounds. The formula used to calculate the fuel economy
from the emission data from a 1972 Federal Test Procedure (FTP) test is:
1, 2423
-' miles per gallon, mpg = .866 (HC) + .429 (CO) + .273 (a>2)
where HC, CO, and CO. are the emissions of HC, CO, and CO. expressed in grams
per mile. This formula is different than the one that was prsented in the
earlier report, and is more precise, due to the inclusion of the HC term
and the fact that the numerator has been modified to more closely reflect
the actual density of the fuel used in the tests.
—' Designates footnotes which can be found on page 35.
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The manner in which the average mpg values for classes of vehicles are
calculated also differs in this report compared to the earlier report. The
results in this report are based on total miles traveled by all vehicles in
the class divided by the total gallons used by all of them. The example given
in footnote —' demonstrates why this is important in attempting to accurately
determine fuel economy. In statistical terms, the harmonic mean of the
data is used rather than the arithmetic mean.
The test procedure from which the fuel economy data are derived is the
same test procedure used for determining exhaust emissions, the 1972 Federal
Test Procedure. This procedure consists of simulating a trip of 7.5 miles
in length on a chassis dynamometer, a device which allows tests simulating
actual driving to be conducted indoors under closely controlled experimental
conditions. The driving cycle used for this procedure represents a mix of
urban and suburban driving including several cruises and speeds up to 57 mph.
'One important feature of this test procedure is the "cold start". The
vehicle is allowed to sit or "soak" 12 hours before the test. As a result,
the engine temperature is about 70°F at the time of the test (much below
its normal operating temperature of 180°-200°F), and the engine is not warmed
up before the test. Other components of the drivetrain are also at about
70°F. Therefore, the results of the test are influenced by the warm-up
characteristics of the engine and vehicle, which have a significant effect
on fuel economy.
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IV. GENERAL FACTORS AFFECTING AUTOMOBILE FUEL ECONOMY
Fuel economy —' in miles per gallon (mpg) is a measure of efficiency. It
is the measure of what you get (miles traveled) for what you put in (gallons
of fuel). Automobile engines produce the work required for operation of the
automobile by burning the fuel in the cylinders of the engine. Part of
the chemical energy in the fuel is converted to useful work done by the engine;
the rest ends up as waste heat. This is why automobiles have hot exhausts and
cooling systems and radiators to get rid of this heat. The ratio of the useful
work delivered by the engine to the total energy in the fuel defines the thermal
efficiency of the engine. Current vehicle engines show thermal efficiencies
between approximately 10 and 30 percent, depending on the engine type, speed,
and load.
Engine efficiency is only indirectly related to the fuel economy of an
automobile because, although engine efficiency is a measure of how well the
engine converts the energy in the fuel to useful work, the total amount of
work required of the engine to drive the automobile depends on the characteristics
of the automobile (engine and vehicle design) and on how the vehicle is
operated. Therefore, the total fuel consumed depends on the engine, the
vehicle, and the operator. Since the fuel economy of the complete automobile
is of most interest, this report uses mpg values to denote fuel economy, and
not any measure of engine efficiency by itself.
A- ENGINE/VEHICLE DESIGN
There are many aspects of automobile design that influence the fuel
economy of automobiles. However, it is not a simple matter to optimize all
of the important factors simultaneously in order to achieve the best fuel
economy.
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Today's vehicle designers are faced with a host of sometimes
conflicting requirements. Since the automobile must sell, it must incorporate
features that appeal to the buying public. Styling, convenience, comfort,
cost, durability, driveability, performance, and fuel economy are among
the factors considered by the buying public. Trends in these consumer
preferences must be anticipated years in advance, since the total automobile
design and development process takes several years before it reaches
production and the consumer.
Within the last five to ten years other requirements have been
added to the list. These requirements, which must also be satisfied by the
vehicle designer, are Federal requirements in the areas of vehicle safety
and exhaust emissions. Today's automobile is a result of compromises,
tradeoffs, and judgments by the vehicle designers as to what combination of
vehicle parameters best suits the overall requirements. Those parameters
which principally influence fuel economy are discussed in this report.
1. Weight
Vehicle weight is the single most important factor affecting passenger
car fuel economy. Sub-compact cars in the lighter inertia weight —' classes
(up to 2,500 pounds) generally achieve double the miles per gallon of full
size cars in the heavier weight classes because a car's engine must do more
work to move a heavy vehicle than a light vehicle. However, this is not the
only reason lighter (smaller) cars achieved better fuel economy. Lighter cars
have also, customarily, been designed to achieve good fuel economy by employing
relatively smaller engines, manual transmissions and fewer accessories.
The difference in fuel economy between light and heavy vehicles has
been increasing as emission controls have become more stringent. The fuel
economy of light vehicles has not been significantly affected by emission
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controls, but heavy cars have realized significant penalties. Figure 1 and
Table 1 illustrate this effect. The solid line shows that, on the average,
the lighter uncontrolled (pre~1968) vehicles achieved much better fuel economy
than the heavy uncontrolled vehicles. The dashed line, representing the
1973 vehicles, indicates the same trend but shows that the fuel economy of
the heavy 1973 vehicles is poorer than the heavy uncontrolled vehicles while
the light 1973 vehicles are slightly better than the light uncontrolled vehicles.
FIGURE 1
FUEL ECONOMY VS. INERTIA WEIGHT
25
20
C3
a.
> 15
O
8
til
_i
UJ
D
10
1957-1967 VEHICLES •-
1973 VEHICLES »•
_L
2000 2500 3000 3500 4000
INERTIA WEIGHT
4500
5000
5500
TABLE 1
FUEL ECONOMY VS.INERTIA WEIGHT
FOR UNCONTROLLED (1957-1967 AVERAGE) AND 1973 VEHICLES
INERTIA WEIGHT
•57 - '67 MPG
73MPG
2000 2250 2500 2750 3000 3500 4000 4500 5000 5500
23.2 21.7 19.1 17.1 15.4 13.5 12.6 11.7 10.9 10.5
23.8 21.9 19.7 17.5 15.6 13.9 10.8 10.1 9.3 8.6
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The weight of most model automobiles has been steadily increasing
in recent years. As can be seen in Figure 2, the most popular standard size
passenger cars have gained about 800 pounds from 1962 to 1973. This trend
in increased weight has also been occurring among intermediate, compact, and
sub-compacts. These weight increases alone have caused a significant drop in
fuel economy of given model vehicles. However, the increased sales percentage
in the lighter weight classes has held the "average" weight increase for all
cars sold in the U.S. to about 25 pounds per year through 1972 as shown in Figure 3.
FIGURE 2
VEHICLE WEIGHT VS. MODEL YEAR
STANDARD SIZE CARS
4500
STANDARD
SIZE
CHEVROLET
m
d. 4000
UJ
m
DC
3500
3000
STANDARD
SIZE
FORD
_L
1958
1962 1966
MODEL YEAR
1970
1973
The amount of increased sales percentage of light vehicles which
will reverse the upward trend in weight and the resulting downward trend
in fuel economy will depend on both the public's buying habits and the auto
industry's ability to improve engine and vehicle efficiency. Increased sales
of convenience devices (e.g., air conditioning, power steering) will continue
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to Increase vehicle weight. Air conditioners for example, add approximately
100 pounds to the weight of a vehicle and cause a 1% to 2% fuel economy
penalty — (depending on type of system and vehicle weight) even when they
are not used to cool the car. Styling can also affect vehicle weight. Vinyl
tops, for example, add weight to the vehicle without performing any function
other than styling. This trend is particularly important for the smaller
cars, since it will lessen the significant fuel economy advantages these cars
now have over larger vehicles which are already extensively equipped with
these optional convenience devices. The techniques chosen by the manufacturers
to meet future safety standards could also have significant impact on the trend
in passenger car weight.
FIGURE 3
VEHICLE WEIGHT VS. MODEL YEAR
DOMESTIC, IMPORTS, AND TOTAL PASSENGER CAR SALES
4000
3500
co
~ 3000
H
X
O
UJ
5
m
§ 2500
o
2000
1500
DOMESTIC
TOTAL U. S.
IMPORTS
1962
1966
1970
1972
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2. Vehicle Size and Shape
The size and shape of the vehicle has an effect on fuel economy
because the automobile has to be pushed through the air as it moves. At the
low speeds experienced during city driving, this air drag effect is small, but
on the highway, at higher speeds, it becomes important. Air drag is related
to the cross-sectional area of the car when viewed from the front. This is
approximately equal to the product of the height and width of the car. This
cross-sectional area is often referred to as "frontal area". The shape of
the car is also important. Even if the frontal areas of two automobiles are
the same the one with the more streamlined shape will have less drag and use
less fuel. It takes more fuel to push a flat faced box at a given speed
than it does to push a streamlined shape, such as the body of a jet plane.
3. Rolling Resistance and Tires
Even if there were no air drag, it would still require power, and
therefore fuel, to drive an automobile, because of rolling resistance. Rolling
resistance is the name given to the resistance due to the tires, bearings, rear
axle, and other rotating components. This resistance is more important to
fuel economy during city driving than is air drag.
Since the rolling resistance due to tires has a significant effect
on overall vehicle rolling resistance, and since the selection and care of
tires are something over which the automobile owner has control, the effect
of tires on fuel economy is important.
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Two aspects of tires are most important to fuel economy—inflation
pressure and type of construction. The correct amount of pressure in tires
varies depending on the type of tire, automobile, and driving conditions.
Information about correct inflation pressure can be found in the owner's
manual for the automobile, and should be followed carefully. Incorrect
inflation pressure can reduce fuel economy and tire life. An underinflated
tire tends to wear out on the edges more quickly and results in a fuel
economy loss. An overinflated tire while producing better fuel economy
tends to wear out in the center faster.
The way in which the tire is made can also affect fuel economy.
The type of tire construction that appears to have the most beneficial effect
on fuel economy is the so-called radial tire. Use of radial tires results
in about a 3% improvement in fuel economy when compared to normal bias
ply tires.
4. Axle Ratio
One of the choices often available to the purchaser of a new
automobile is that of axle ratio. This term refers to the number of times the
driveshaft turns for each time the rear wheels turn. Numerically this number
ranges from about 2.50 to over 4.00 for current automobiles. Generally, a
numerically lower axle ratio will result in better fuel economy, compared
to a higher value because, although it produces the same power, the engine
runs slower for any given vehicle speed and therefore has less internal
friction to overcome. Also, for a given power output (vehicle speed) the
engine is operating more efficiently at the lower engine speed because of
reduced throttling losses. For example, changing the axle ratio 10%
(e.g. from about 3.0 to 2.7) can improve fuel economy by about 2% to 5%.
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Another way to obtain the benefits of making the engine run slower
for a given vehicle is the overdrive feature with which some automobiles are
equipped. In essence this is another gear to shift into once the vehicle is up
to cruising speed on the road, reducing engine speed and improving economy. Fuel
economy gains of more than 10% during cruising conditions are possible with
overdrives. However, despite its merits, overdrive has fallen into disuse. This
may be due in part to increased driving in urban areas where overdrive is not
used, the greater initial cost, and greater use of automatic transmissions.
5. Convenience Devices
Of the many convenience devices available to the new car buyer,
the following can have a negative effect on fuel economy.
1. air conditioning 5. power seats
2. automatic transmission 6. power windows
3. power steering 7. power sunroof
4. power brakes
All of these devices can cause fuel economy penalties in as much
as they all add to the vehicle weight. In addition, some of the devices
consume significant amounts of energy directly during use. Two of the more
important devices, air conditioning and automatic transmissions, are
discussed below.
Air conditioning has a two-fold effect on fuel economy. As
discussed earlier, the addition of the approximate 100 pounds weight of
the system causes a 1% to 2% penalty. A much larger penalty is suffered
when the air conditioner is actually running, since the engine is required
to produce additional power to drive the compressor. The effect on fuel
economy will vary depending on the ambient temperature and the type of
driving. Stop-and-go driving in hot weather can result in a 20% penalty
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if the air conditioning system is turned on. An "average" loss associated
with the use of air conditioning is about 9%. Obviously, this loss in fuel
economy and the resultant increased gasoline consumption tends to occur
during the summer months, when recent fuel shortages were most critical.
The automatic transmission has often been associated with significant
fuel economy penalties. When other aspects of vehicle design remain constant,
the use of an automatic transmission can result in a fuel economy loss of
up to 15%. However, the data in the earlier EPA report and in other studies
failed to fully consider the impact of transmission type on exhaust emission
controls. Vehicles with manual transmissions sometimes require more severe
(e.g. more spark retard) engine calibration to meet a given level of emission
control than do vehicles with automatic transmissions, since the throttle
movement required during the shifting of a manual transmission equipped vehicle
tends to increase HC emissions.
Analysis of the fuel economy data from vehicles designed to meet the
1973 Federal emission standards shows that, on the average, automatic
transmission equipped vehicles show only slightly worse fuel economy (2% loss)
than vehicles equipped with manual transmissions. Greater fuel economy
advantage (6%) is seen for the manual transmission in the lighter weight
classes. This may be due in part to the use of less sophisticated automatic
transmissions in these light weight categories and the increased use of the
energy consuming torque converter in these vehicles which tend to have low
power- to-weight ratios.
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6. Engine Design
The design of a vehicle's engine can have a significant effect on
fuel economy. This is particularly true in view of the different techniques
various manufacturers have chosen to reduce manufacturing cost, meet emission
standards, reduce octane requirements and produce additional power.
One manufacturer may choose to meet emission standards by the use of
control techniques such as ignition spark retard, which will reduce fuel
economy; another manufacturer may use fuel injection to meet the same
standards with a fuel economy improvement. The manufacturing cost of emission
control systems which do not reduce fuel economy is, however, generally higher
than the cost of systems which sacrifice fuel economy for low emissions,
hence fuel economy tends to be sacrificed by automobile manufacturers in
favor of lower vehicle sale prices.
Many passenger cars currently sold in the U.S. have lower compression
ratios now than prior to 1971. This trend has tended to reduce fuel economy
somewhat. The reduction in average compression ratio from approximately 9.3:1
to 8.3:1 has reduced fuel economy about 3.5%. This change, however, has also
reduced the octane requirements of engines from 94 octane (regular leaded fuel)
to 91 octane (presently low lead). The customer can usually purchase these
low lead fuels for one cent per gallon less than "regular gasoline". This
can result in approximately a 2.5% fuel cost savings which makes up most of
the cost penalty associated with the compression ratio reduction, although
the fuel economy penalty (and the associated increased consumption of
petroleum) is still present.
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Techniques to increase compression ratio without increasing the
engine octane requirements could result in significant fuel economy improvements
without increasing fuel costs. Such techniques involving the use of proportional
exhaust gas recirculation systems and high swirl combustion chambers have been
investigated by the industry and may be available to the public in the future.
The size (horsepower or displacement, which are directly related
for most conventional engines) of the engine can also have a significant
effect on fuel economy. When two vehicles are identical in all other respects,
the vehicle with the smaller engine will usually show better fuel economy.
This is because spark ignition engines tend to be more efficient when
operated at a higher percentage of full load power. For a given driving
condition, two vehicles which are identical except for their engines will
have equal horsepower requirements. The vehicle with the smaller engine,
however, will have to operate nearer full load than the vehicle with the
larger engine, thus delivering better fuel economy. But when the power
required to drive the vehicles is so large, or the engine's maximum available
power is so low, that the engine in one of the vehicles is operating at
full load, then the larger engine may deliver better fuel economy. This is
because most engines are inefficient when operated at full load, where some
fuel is intentionally wasted in order to obtain maximum utilization of the
air passing through the engine. The optimum load for a given engine depends
on many engine parameters (ignition timing, carburetor calibration, etc.)
and cannot be generalized.
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7. Control of Vehicle/Engine Design Parameters to Achieve
Improved Fuel Economy
While engine displacement and horsepower are directly related for
most passenger car engines today, this does not have to be the case. Several
different techniques are available to increase the horsepower of an engine by
making high pressure intake air available. This can be done with turbochargers
and superchargers. Efforts to improve fuel economy by restricting the allowable
horsepower could 'prevent the development of engine concepts which result in
good fuel economy and higher horsepower simultaneously.
Controlling the displacement allowable for passenger cars would
be a more logical approach; however, even that would be unfair to manufacturers
who have the talent to develop engines that are highly efficient without
being small. The most obvious example of how different engine designs can
cause different efficiency for a given displacement can be seen in the case
of the Mercedes 220 series automobiles.
Mercedes builds two 1973 models that fall in the same weight class
and have the same size engines. Yet one model, the 220D, delivers 24 mpg in
urban/surburban driving while the other model, the 220 gasoline, delivers
only 13 mpg. Although these two models were tested at the same weight and
with the same transmission type, the fuel economy of one is 85% better than
the other. The 220D model uses a Diesel engine which delivers much better
fuel economy than conventional gasoline engines of equivalent displacement.
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The use of non-conventional engines in the market place will
essentially eliminate the correlation between horsepower and fuel economy or
displacement and fuel economy. Differences in engine design also make
impractical the use of weight as a possible control variable. Some
2,750 pound vehicles powered by rotary engines deliver worse fuel economy
than many 4,000 pound vehicles with conventional engines.
Because design differences in engines can have such a pronounced
effect on fuel economy, there is no simple and equitable way to improve
fuel economy of passenger cars by restricting the design (e.g. horsepower
limit, displacement limit, weight limit) of the vehicle. Any control
measure, to achieve its objective in the least limiting way in terms of
stifling innovation, should be based directly on the fuel economy achieved,
in terms of fuel required for miles driven on a standardized test.
8. Alternative Engines
Alternatives to the conventional gasoline engine may be produced in
large numbers in the future and the use of alternative engines could have
a significant impact on fuel economy. However, just because an engine is
different than a conventional engine does not mean its fuel economy will be
better. While the development of alternate engines is continuing and progress
in the area of fuel economy will probably be made, the same is also true for
the conventional engine. As shown in Table 2, as of today, some alternate
engines have demonstrated improved fuel economy over the conventional engine,
some demonstrate equivalent fuel economy, and some demonstate inferior
fuel economy.
Of the available alternatives, the diesel engine offers the
inaTr-iimnn potential for improved vehicle fuel economy. Although it has been
in commercial production for over 50 years, it is imported into this country
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21
in very small quantities, and no domestic manufacturer has indicated an intention
of producing a diesel-powered vehicle for domestic sales. However, a
second foreign manufacturer has indicated that he will import a diesel-powered
vehicle beginning in 1974.
TABLE 2
FUEL ECONOMY OF VEHICLES EQUIPPED WITH ALTERNATIVE ENGINES
EXPECTED TO BE IN USE IN THE NEAR FUTURE.
% CHANGE COMPARED TO AVERAGE 1973 VEHICLE OF SAME WEIGHT
WORSE
1. ROTARY: 35% LOSS
EQUIVALENT
1. PRE-CHAMBER
STRATIFIED
CHARGE (HONDA
CVCC)
BETTER
1. DIESEL: 40% TO 70% GAIN
2. CONVENTIONAL ENGINE
EQUIPPED WITH CATALYST:
0% TO 15% GAIN
3. OPEN CHAMBER STRATIFIED
CHARGE (PROCO): 12% GAIN
9. Emission Controls
Fuel economy penalties brought about by emission control devices
have been reported by many different sources. The idea expressed in many
reports is that "everyone knows" fuel economy has suffered because of emission
control. Usually a percent penalty, one number, is given as "the penalty".
Such reports are, however, generally not supported by a statistically
significant data base.
EPA studies involving several thousand tests of both uncontrolled
(pre-1968) and controlled cars indicate that the effect of emission controls
on fuel economy has not been the same for all cars. Some models have
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22
realized severe penalties, but other models have realized improvements. A
definite trend can be seen from the data. Figure 4 shows that the change in
fuel economy between 1973 cars and uncontrolled cars is strongly dependent
on the weight of the car. 1973 vehicles in the lighter inertia weight
categories (up to 3,500 pounds) show slightly better fuel economy than
uncontrolled cars, but vehicles in the heavier categories (4,000 pounds
and above) have demonstrated significant penalties, as much as 18% for
the heaviest weight class. These figures include the impact of changes
in compression ratio.
FIGURE 4
CHANGE IN FUEL ECONOMY
BETWEEN '57-'67 AVE. AND 73 BY INERTIA WEIGHT CLASS
2000 2500 3000 3500 4000 4500 5000 5500 INERTIA WEIGHT
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23
Table 3 presents the same information shown in Figure 4 in the
tabular form. The percent change shown for each weight class was determined
from the average fuel economy of all cars tested in that class. Trends
may have been different for individual models,
TABLE 3
CHANGE IN FUEL ECONOMY DUE TO EMISSION CONTROLS
1973 VEHICLES COMPARED TO UNCONTROLLED VEHICLES
INERTIA WEIGHT CLASS % CHANGE
2000 + 2.6
2250 + .9
2500 + 3.1
2750 + 2.3
3000 + 1.3
3500 + 3.0
4000 -14.3
4500 -13.7
5000 -14.7
5500 -18.1
The reason for the dramatic difference in fuel economy change between
the light and heavy passenger cars appears to be due in part to the difference
in the degree of control required to meet the 1973 oxides of nitrogen (NOx)
exhaust emission standard, 3.0 grams per mile. The lighter cars need less
control to meet this standard than do the heavy cars because their smaller
power requirement results in a lower volume flow of their exhaust gas and
therefore lower mass emissions. Thus, while techniques used by the industry
to control NOx (e.g. spark retard and non-proportional exhaust gas
recirculation, EGR) have adversely affected fuel economy, many light cars
need little or no NOx control to meet the standard and therefore they have
not realized this fuel economy penalty. In fact, since many light cars
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24
use emission control techniques (e.g. mixture enleanment or more precise
fuel management through the use of fuel injection) which can reduce HC
and CO emissions while improving fuel economy and need little additional
NOx control, slight improvements in fuel economy are found in the lighter
weight classes. However, the step-change between the 3,500 and 4,000 pound
weight classes is not fully understood at this time since the. same change
is demonstrated for other model years as well. The EPA will continue to
investigate this difference.
Because of this difference in fuel economy penalty, the average penalty
realized by the driving public will depend on which cars the public buys.
If more heavy cars are sold the penalty will be severe (up to 15%). This
penalty coupled with the already poorer fuel economy of heavy cars would result
in a drastic increase in gasoline demand. If, however, more light cars are
sold there will be less penalty associated with emission controls, and gasoline
demand would be sharply reduced since light cars also get better fuel economy
than heavy cars. If the public buys light and heavy cars in the same pro-
portions as they bought them in 1972 then the "average" penalty for the 1973
models will be 10.1%, — including the 3.5% loss due to compression ratio
changes discussed earlier.
The effect of future emission standards on fuel economy has been
considered by EPA in making decisions on the feasibility of the future
standards. While there can be disagreement on this issue, it appears that
the changes in engine/vehicle design required to meet the HC and CO levels
will result in improved fuel economy. Much of this improvement will be due
to the rapid release of the choke which will be made possible through the use
of quick heat intake manifolds and higher energy ignition systems. When a
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25
vehicle is operated with the choke on, the fuel economy is poor because the
mixture delivered to the engine is richer then required for optimum economy.
The choke is necessary only when the vehicle is being started and warmed up.
When choking requirements are reduced, fuel economy is improved during vehicle
warm-up.
Some of the improvements expected on vehicles designed to meet
future standards will also be due to the use of improved EGR systems and
optimized ignition timing which will allow heavy cars to gain back some of
the economy lost in 1973. General Motors data on prototype vehicles indicates
that the fuel economy of their vehicles designed to meet the 1975 and 1976
interim standards will be up to 15% better than 1973 vehicles.
B. Vehicle Operation and Use
The manner in which a vehicle is used has a significant effect
on vehicle fuel economy. This effect can be as, or more, important than the
design of the vehicle and engine itself. It is also one aspect of vehicle
fuel economy over which the vehicle operator has control throughout the
vehicle's life and not only at the time of purchase.
1. Vehicle Speed and Trip Length
Vehicle speed has a significant effect on fuel economy. The
energy required to drive a vehicle a given distance goes up as speed increases.
The impact of air drag and rolling resistance on fuel economy, plus the way
in which the engine efficiency varies with speed and load, combine to produce
the results for a typical domestic automobile shown in Figure 5 and Table 4.
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26
FIGURES
FUEL ECONOMY VS. VEHICLE SPEED
C9
a.
20
O
o
111
u.
15
STEADY CRUISING
URBAN DRIVING (STOP + GO, AVG. MPH=20)
20
30
40 50
VEHICLE SPEED (MPH)
60
70
TABLE 4
FUEL ECONOMY VS.VEHICLE SPEED
URBAN DRIVING
CRUISE
SPEED
20 MPH
20 MPH
30 MPH
40 MPH
50 MPH
60 MPH
70 MPH
FUEL ECONOMY
10MPG
16.5 MPG
22.0 MPG
22.5 MPG
21. 5 MPG
19.5 MPG
17.3 MPG
This figure and table show how fuel economy is affected by the
steady cruise speed — . Two things can be seen from this information.
The best fuel economy occurs at a steady speed of between 30 and 40 miles
per hour. While interesting, it is not of much practical value because
few trips are made at a constant speed between 30 and 40 miles per hour. The
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27
most Important knowledge to be gained from this information is the effect
of high speeds. At a cruise speed of 70 miles per hour, the fuel economy
is significantly worse than at 60 or 50. Cruising at 60 instead of 70 miles
per hour improves economy about 15%. Cruising at 50 instead of 70 miles
per hour increases the savings to about 25%.
Trip length also has a significant effect on fuel economy.
Figure 6 shows that the fuel economy achieved during an urban trip is strongly
dependent on the length of the trip. Short trips result in poor fuel economy.
The engine is less efficient while it is warming-up, due primarily to fuel
enrichment (choking needed during start-up) and engine and driveline
friction which are higher when the vehicle is cold.
FIGURE 6
FUEL ECONOMY VS. TRIP LENGTH
10
TRIP LENGTH (MILES)
15
20
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28
Figure 6 shows that the difference in fuel economy between
short trips and long trips is dramatic. The vehicle used to develop this data
had a fully warmed-up fuel economy of 13.5 mpg under the same driving
conditions. Driven on a ten-mile trip the economy would drop to about 11
mpg and driven on a one-half-mile trip the economy would be only 5 mpg.
Figure 6 applies only to trips that are started with a "cold"
engine. The engine can be considered "cold" if the vehicle has been parked
overnight or all day long. The same trend would also apply to engines which
are warm when the trip is started, but the fuel economy for a short trip would
be much better than had the engine been cold at the beginning of the trip.
Figure 6 must be interpreted carefully. The graph indicates
that a driver could get better fuel economy by taking a longer route to his
designation. This is true but this is also false economy. The mpg value
would "Tie higher but the total fuel consumed would also be higher. The total
amount of fuel consumed in going from point A to point B is obviously more
important than the mpg value obtained between points A and B.
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29
2. Other Driving and Maintenance Habits
The manner in which an automobile is driven can influence its fuel
economy. The driver who habitually accelerates away from a stop as fast
as he can uses up to 15% more fuel, compared to a driver who uses a moderate
acceleration. Other driving habits that can help fuel economy are anticipating
stoplights and slowing down gradually, driving smoothly and making as few as
possible unnecessary speedups and slowdowns, and keeping idle time to a
minimum. At a speed of 50 mph, one speed change per mile can result in up to
a 25% increase in fuel consumption. Prolonged periods of idle should also be
avoided since an idling automobile delivers zero miles per gallon fuel economy.
Automobiles, like other machines, require maintenance to operate
properly. Lack of, or improper, maintenance can hurt fuel economy. The proper
maintenance items and frequency are described in the owner's manual and should
be followed carefully. Areas requiring periodic maintenance that can affect
fuel economy are: air filters, the ignition system (spark plugs, distributor
points, and ignition timing), carburetor and proper air-fuel mixing, cylinder
compression, and lubrication. If any or all of these areas are not in proper
working order or the correct part is not used, fuel economy will suffer.
Keeping an automobile tuned up can on the average improve fuel economy 6%,
compared to an untuned automobile. However, an individual vehicle which is
grossly maladjusted or unmaintained (e.g., spark plug misfiring, clogged
air filters, improper carburetor adjustment) can suffer a significantly
worse fuel economy penalty of more than 20%.
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30
3. Weather and Road Conditions
The weather in which an automobile is operated can have an effect
on fuel economy. Generally, the colder the temperature the worse the fuel
economy. This is due to two effects. When it is cold, it takes longer for
the engine and drivetrain to warm up, thus hurting fuel economy. However,
even when the engine and drivetrain are warm, colder weather generally tends
to reduce fuel economy. This effect is about a 2% loss in fuel economy in
each 10°F drop in temperature at 50 miles per hour. Many current automobiles,
because of emission control requirements, have provisions for heating the
intake air. This helps to reduce the adverse effect of low temperature on
fuel economy.
The wind can also have an effect on fuel economy. Cruising at
50 miles an hour into a 20 miles per hour headwind results in fuel economy
much closer to what would be obtained cruising at 70 miles per hour with
no wind. This is because of the increased air drag due to the wind.
The elevation at which an automobile is operated will also affect
fuel economy. At high altitudes, current design carburetors get "fooled"
and deliver more fuel to the engine, compared to the amount of air, than they
should. The vacuum advance feature of the ignition system also fails to
function properly at high altitude. This can reduce fuel economy up to 15%
at 4000 feet elevation. Modifications to carburetors and ignition systems
can eliminate the high altitude fuel economy penalty but the vehicle will
then drive poorly at low altitude. Altitude compensated carburetors and
ignition systems are currently being developed by several automobile manu-
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31
facturers. If these systems are put into production in the future, fuel
economy penalties will not be experienced at high altitudes. Most vehicles
currently equipped with fuel injection already provide some compensation
for altitude which reduces the penalty.
The type of road surface and the grade of the road have an effect
on fuel economy. The poorer the road, the worse the fuel economy. At
40 miles per hour cruise, badly broken and patched asphalt causes about
a 15% fuel economy penalty, compared to a good smooth road. Gravel causes
a 35% penalty, and dry sand has a 45% penalty. Dirt roads probably fall
somewhere between the bad asphalt and the gravel.
Going uphill reduces fuel economy because the engine has to supply
power not only to move the automobile along the road, but also to lift it
to the top of the hill. The "grade" of a road is a measure of how steep it
is. The maximum grade on most interstate highways is about 5 to 7 percent.
Going 50 miles per hour up a 7% grade results in a fuel economy penalty of
55%, compared to going 50 mph on a flat road. On a 3% grade this penalty is
about 32%.
V. TRENDS IN AUTOMOBILE FUEL ECONOMY
Year-to-year trends in fuel economy are the combined effects of trends
in all of the parameters which affect fuel economy. For any given model year
the average fuel economy will depend on the design characteristic of vehicles
that are sold, which in turn depends, in part, on what emission and safety
standards are in effect and on consumer preference as expressed through buying
habits.
By using sales and weight data from vehicle registration lists and fuel
economy data from the EPA Federal Test Procedure, a "sales-weighted" average
fuel economy —' for the model years 1957 through 1973 has been calculated. Figure 7
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32
shows the trend in sales-weighted fuel economy. The same data are presented in
Table 5. While market changes (e.g. the penetration of the subcompact car in
the early '60's) have had significant effects on individual model years, the
general trend has been toward poorer fuel economy. It can be seen from Figure 7
that the loss in average fuel economy during the last 12 years (1962-1973) has
been about 16%. Prior to 1968 and the imposition of Federal emission standards,
this loss was due largely to vehicle weight increases and the associated changes
in engine size, and the increased usage of convenience devices. This trend towards
worse fuel economy is slightly greater for the model years after 1967 which were
subject to exhaust emission standards, and during which there was an even greater
rate of increase in the usage of convenience devices and the trend toward higher
vehicle weight. However, the increase in average vehicle weight (more than 350
pounds) and the associated changes in engine size over the total 12 year period
alone have accounted for about 1/2 of the total loss.
FIGURE 7
SALES WEIGHTED FUEL ECONOMY
VS. MODEL YEAR
16
O 15
O
O
* 14
UJ
D
u.
0 13
UJ
I12
CO
3 11
10
1973 DATA
ARE ESTIMATED
1958 1960 1962 1964 1966 1968 1970 1972 1974
YEAR
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33
TABLE 5
SALES WEIGHTED FUEL ECONOMY BY MODEL YEAR
SALES WEIGHTED
MODEL YEAR MILES PER GALLON
1957 13.67
1958 14.07
1959 13.85
1960 13.36
1961 13.55
1962 13.96
1963 12.62
1964 13.49
1965 12.98
1966 12.95
1967 12.86
1968 12.44
1969 12.21
1970 12.51
1971 12.21
1972 12.03
1973 11.67
76 <74
Another way to consider fuel economy trends, that relates more directly
to total fuel consumption, is to examine the fuel economy for all cars on the
road, not just the new models. This is the basis of the true national average
fuel economy figures which are reported annually by the Department of Transportation.
DOT's values are calculated from total miles travelled by passenger cars and
total gallons of fuel sold to passenger cars in each calendar year. This is
shown as the upper curve in Figure 8. The nationwide average fuel economy for
all cars on the road can also be determined using the EPA test results, if the
make-up of the total passenger car population in any one calender year is known.
Using registration data, annual vehicle mileage as a function of age, and vehicle
attrition rate information, a "national average" fuel economy has been calculated
for several calender years. The trend in national average fuel economy as
determined by this method is shown in the lower curve in Figure 8.
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34
FIGURES
NATIONAL AVERAGE FUEL ECONOMY
VS. CALENDAR YEAR
O
Q.
O
U
UJ
14
13
u,
III
O --
< 12
cc
11
r°
O DOT DATA. CALENDAR
YEAR BASIS (CY)
A EPA/FTP DATA, MODEL
YEAR BASIS (MY)
1966
1967 1968 1969 1970 1971 1972 1973 CY
1966 1967 1968 1969 1970 1971 1972 1973 MY
Both sets of data show the same trend, a downward shift in national
average fuel economy of from 3% to 6% depending on the years chosen for
comparison. In addition to showing the same trend, it can be seen that the
fuel economy values based on the 1972 Federal Test Procedure results correlate
closely with the absolute value of the DOT results, indicating the driving
cycle used for the Federal emission test is a good representation of customer
average driving. A modification (the inclusion of a hot start and about three
additional miles of operation) being made to the Federal Driving Cycle for
the 1975 and later model years results in nearly perfect correlation with
DOT's values.
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35
Footnotes:
I/ A more detailed discussion of the derivation and use of the equation can
be found in Appendix A.
2f Suppose a motorist took a trip of 600 miles and used three tanks of gaso-
line. For the first 200 mile segment he used 10 gallons, in the second
200 mile segment he used 20 gallons, and for the third 200 mile segment he
used 18 gallons. If he just averages the individual mpg results he gets
the wrong answer. The individual fuel economy values for the three seg-
ments are 20 mpg (200/10), 10 mpg (200/20) and 11.1 mpg (200/18). The
simple average is (20 + 10 + ll.l)/3 = 13.7 mpg. But the trip was 600
miles long and 10 + 20 + 18 « 48 gallons were used, so the trip fuel
economy was 600/48 =12.5 mpg, not 13.7.
i. .
3/ Fuel economy should not be confused with fuel consumption which is
expressed in terms of gallons of fuel consumed per mile. One is the
inverse of the other. A certain percentage increase or decrease in fuel
economy does not equal the same percentage decrease or increase in fuel
consumption. For example, one car getting 20 MPG has 33% better fuel
economy than one with 15 MPG. However its fuel consumption is 25% less.
The two terms cannot be used interchangeably.
4/ The term "inertia weight" refers to the test weight of the vehicle that
was simulated on the chassis dynamometer during the emission tests. Inertia
weight corresponds to the weight of the automobile with a full tank of fuel
and two passengers. These classes range from 1750 to 5500 pounds for cars
tested by EPA.
5/ Unless otherwise noted, the losses and gains in fuel economy discussed in
this report refer to urban/suburban driving and not to steady cruise driving.
However, changes in vehicle design or operation which affect urban/
suburban fuel economy will have the same relative effect on steady
cruise fuel economy.
6/ The calculation of the sales weighted average fuel economy loss due to
emissions controls assumes the same market share for the various weight
classes for both 1973 and pre-1968. This is done to avoid the possible
confounding effects of fuel economy changes due solely to shifts towards
heavier or lighter cars being attributed to emission controls. The loss was
calculated based on the harmonic means of the fuel economy data or, in other
words, based on average fuel consumption data. If the calculations had been
based on the average of the fuel economy data, the loss due to emission
controls would have been shown to be significantly less.
Tj The steady cruise fuel economy at a given speed should not be confused with
the fuel economy obtained during stop and go driving but at the same average
speed. This difference is shown in Figure 5. The fuel economy achieved during
actual "cruising" will be less (relative to urban driving) than that indicated
by Figure 5 because of the many speed changes made (passing other cars),
wind conditions, hills, etc.
s.
8/ Sales weighted average fuel economy is the average fuel economy of all cars
sold in a given model year, taking into account the number of cars sold in
a given weight class.
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36
Appendix A
The equation used to calculate the fuel economy of a vehicle, in miles per
gallon (mpg), from data gathered during a 1972 Federal Emission Test is of the
following form:
mpe ..grams of carbon/ gallon of fuel
grains of carbon in exhaust/mile (A-l)
mpg = _ (grams/gallon) _ _____ _ ,A_2)
F8 (Kx) (grams HC/mi) + (K2) (grams CO/mi) + (K3) (grams C02/mi) k '
where:
Kl = carbon weight fraction of gasoline or unburned HC
(mol. wt. C) / (mol. wt. CHi.85) = *866
K2 = carbon weight fraction of CO, (mol. wt. C) / (mol. wt. CO) - .429
K3 - carbon weight fraction of C02, (mol. wt. C) / (mol. wt. C02) » .273
grams/gallon - mean density of Indolene 30 test fuel = 2798
substituting:
_ .866 (2798) _
.866 (gpm HC) + .429 (gpm CO) + .273 (gpm C02) U~J;
_ 2423 _
.866 (gpm HC) + .429 (gpm CO) +.273 (gpm C02)
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37
Appendix B
Fuel Economy in Miles per Gallon for Various
Model Years and Inertia Weight Categories
(—indicates no data)
Inertia Weight
Model
Year 1750
57 —
58 —
59 —
60 —
61 —
62 —
63 —
64 —
65 —
66 —
67
68 —
69 —
70 —
71 27.2
72 —
73 24.8
74 —
75 —
57-67
Aver .
2000
26.4
25.3
28.6
20
29
25
23
22
23
20
22
19
.4
.4
.8
.2
.8
.8
.9
.6
.3
22.2
23
22
23
23
24
23
.4
.6
.0
.8
.1
.2
2250
18.2
—
—
—
19.5
—
—
—
25.7
20.5
20.3
19.3
21.4
21.9
21.9
21.4
20.1
21.7
2500
22.3
20.9
—
—
—
—
12.7
—
18.5
18.8
17.5
19.3
19.6
19.7
18.7
17.4
19.1
2750
13.2
24.5
16.3
18.0
16.1
17.3
18.3
14.9
18.7
19.7
—
18.5
18.3
20.0
17.5
17.7
16.6
17.1
3000
15.2
—
17.2
16.3
14.7
16.2
15.2
14.6
15.9
15.6
15.4
15.9
14.8
14.4
15.6
14.8
—
15.4
3500
14.7
13.6
15.0
15.7
11.4
13.0
12.6
13.7
13.7
13.9
13.1
13.3
13.3
13.3
12.2
13.3
13.9
13.7
14.3
13.5
4000
13.0
15.2
13.2
12.4
14.0
13.8
12.0
12.9
12.3
12.3
12.1
12.0
11.9
12.0
11.7
11.1
10.8
10.8
—
12.6
4500
12.5
12.7
10.8
10.5
12.6
11.1
11.4
11.7
12.1
11.6
11.3
11.3
10.9
10.7
10.7
10.1
9.6
10.1
11.7
5000
8.6
13.8
10.9
10.6
10.8
10.6
11.0
10.3
11.3
11.2
9.5
9.1
10.1
9.6
9.6
9.3
9.1
9.6
10.9
5500
12.5
—
—
—
—
—
—
9.3
10.3
—
10.8
9.9
10.9
9.3
8.6
8.2
8.4
10.5
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38
Appendix C
ANONTATED BIBLIOGRAPHY
Further Information about automobile fuel economy can be found In the.following
references:
1. C. E. Schffler and G.W. Niepoth, "Customer Fuel Economy Estimated from
Engineering Tests", SAE paper 650861, November 1965.
This paper discusses some of the factors that Influence fuel economy. Among
the specific factors treated are the effect of how "hard" the vehicle is driven, and
the significant effect that short trips, cold engines have on fuel economy.
2. "Weight Trends of Passenger Vehicles", The Aerospace Corporation, El Segunda,
California, October, 1973.
This report contains Information about the trends in vehicle weight over
the time period 1958 through 1972. Data are presented on the average
(sales weighted) weight during the 1958-1972 time period, weight trends of specific
automobile types, such as subcompact cars, compact cars, intermediate cars and
standard cars, weight trends of certain specific model automobiles, and trends in
in accessory and convenience device installation.
3. Paul J. Claffey "Running Costs of Motor Vehicles as Affected by Road Design
and Traffic", National Cooperative Highway Research Program Report 111 , 1971.
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39
-2-
This report includes data on how road type, grade, curvature, traffic
density and vehicle speed affect fuel economy. This report also discusses
the fuel economy of trucks and contains information concerning some of the
other operating costs of motor vehicles, such as oil consumption.
4. G. J. Huebner Jr. "General Factors Affecting Vehicle Fuel Consumption",
SAE paper, May 1973.
This paper contains information about the effect of some vehicle parameters,
such as axle ratio, compression ratio, and engine size on,fuel economy.
Also presented are some data on the steady state fuel economy of three
different kinds (size and weight) of vehicles as a function of speed.
,*
The effect of tire type is also discussed in this paper.
5. T. C. Austin and K. H. Hellman "Passenger Car Fuel Economy - Trends and
Influencing Factors", SAE paper 730790, September 1973.
This paper contains information on trends in fuel economy from 1957 to
1973. Fuel economy data are presented on the basis of the 1972 Federal Test
Procedure. Sales weighted fuel economy and national average fuel economy
are presented and compared to other references. The effect of various
engine and vehicle parameters are quantified by use of a regression analysis«
The effect of emissions controls on fuel economy is also discussed. Much of
the information in this paper was used in the preparation of this EPA report
on automobile fuel economy.
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