EPA-AA-CAB-77-1 Comparison of EPA Measured Fuel Economy with the Mileage Guide James A. Rutherford August 1977 Characterization and Applications Branch Emission Control Technology Division Office of Mobile Source Air Pollution Control Office of Air and Waste Management U.S. Environmental Protection Agency ------- -2- Background The Gas Mileage Guide presented by EPA in conjunction with FEA is a tool for comparing new cars on the basis of fuel economy. Test sequences which produce the figures for the guide are precisely defined in an effort to provide uniformity in evaluation and more scientifically comparable results. Prototype vehicles at 4,000 miles are driven on a dynamometer by professional drivers in a 75° F environment. Starting, stopping, acceleration, and deceleration within the city and highway cycles are intended to be representative of these modes of operation by consumers. It would not be expected that an owner calculating gas mileage for his car would get the exact figure shown in the guide although the discrepancy should not be too great. The difference between an owner determined gas mileage and the guide value may be considered to contain two major components. The first is the difference between the owner's determination of gas mileage and the figures that would result if his car were put through the test sequences used by EPA. These differences include the specific type of driving, the ambient temperature, the vehicle engine temperature, etc. The second is the difference between these tests run on the consumer's in-use vehicle and the published figures in the guide for that specific type of vehicle. These differences include prototype/ production differences as x^ell as differences in specific vehicle configuration such as axle ratio, test weight, tires, etc. This report will focus upon the second of the two components. Data Data utilized in this report come from the Fiscal Year 1975 Emission Factor Program. The program includes testing of 2200 vehicles from 1966 through 1976 model years. Consumer owned, in-use vehicles were selected in seven cities based upon sales weighting for the determination of make and model and based upon vehicle miles traveled for the determination of model year characteristics. Information in this report is based upon the model years 1975 and 1976. City fuel economy is calculated for each vehicle from data obtained in the 1975 Federal Test Procedure via the carbon balance method. Highway fuel economy results were obtained on a subset of the vehicles via the Federal Highway Fuel Economy Test. Various classification parameters (e.g., engine size, transmission type, etc.) were recorded at reception of the vehicle for testing. The vehicle owners were asked to complete a questionaire which included information about vehicle use and maintenance. Of 842 model year 1975 and 1976 vehicles included in FY75 EFP, 235 had Highway Fuel Economy Tests performed. City and highway fuel economies as they appear in the 1975 and 1976 Gas Mileage Guides were used for all but thirty of the vehicles which could not be located in the guides. ------- -3- Approach In examining the discrepancies between EF values and guide values for fuel economy, the prima facie approach would be to consider one value minus the other. However, a difference of 3 miles per gallon would probably be more important when dealing with values around 8 miles per gallon than around 30 miles per gallon. This would lead to a consideration of some relative measure. In this report it is assumed that both absolute and relative measures are of interest. The analyses are performed with both types of measures thus providing the possibility of determining whether they lead to consistent conclusions. Absolute differences were calculated as Emission Factor values minus Gas Mileage Guide values (i.e., the result from HFET minus the guide highway fuel economy and the result from FTP minus the guide city fuel economy for each vehicle). Relative measures were calculated as EF value as percent of guide value (i.e., HFET divided by guide highway fuel economy multiplied by 100 and FTP divided by guide city fuel economy multiplied by 100). Thus, if the EF value were less than the guide value the difference would be negative and the percent would be something less than 100, if the EF value x^ere equal to the guide value the difference would be zero and the percent 100, and if the EF were greater than the guide value the difference would be positive and the percent something greater than 100. j- That the resultant differences were not all zero and that there was a great deal of variability will be presented later. Beyond the overall results it is of interest to determine whether the differences show any systematic relationship to various vehicle classification and maintenance factors. Due to the nature of the measurements being utilized, normal theory statistical approaches do not seem appropriate. The non-parametric method of choice for determining whether vehicle classification and maintenance factors have significant statistical effects upon the EF/guide fuel economy differences and percents is the analysis of variance test applied to ranks (termed the Kruskall-Wallis test). In light of the stipulated purpose of the Fuel Economy Guide being the comparison of vehicles, it is also of interest to consider the question of how the ranking of the fuel economies of vehicles compares between EF and guide values. For this purpose a nonparametric correlation measure is utilized. Due to the large number of tied observations the Goodman - Kruskal Gamma is used rather than the more frequently seen Kendall's Tau. The G-K Gamma is similar to a normal theory correlation coefficient in that its possible values range from -1 to +1. As in normal theory a value of zero indicates independence while values approaching +1 indicate strong agreement. From the G-K Gamma an estimate of the probability of concordance is calculated where the probability of concordance is defined as the probability that, for two vehicles drawn at random from the appropriate stratum, if one of the measures ranks one vehicle above the other then the other measure will rank them in the same order. ------- The interaction of fuel economy and regulated emissions has been a topic of much interest as a result of increasing demand for fuel and more stringent regulations. The analysis of fuel economies in this report are duplicated for that subset of vehicles which are within the national stipulated limits for new cars on HC, CO and NOx for model years 1975 and 1976. Results Of the 842 1975 and 1976 model year vehicles in the data, 398 were within the emission limits for HC, CO and NOx. Thirteen of these passed vehicles were among the original thirty not appearing in the Gas Mileage Guides. Percents (EF as percent of guide value) and differences (EF minus guide value) were calculated for each available vehicle for highway and city fuel economies. A rough presentation of the results of these calculations appears in Table 1. Using the arbitrary figures of a difference of less than two or within ten percent as non-significant differences it is seen that about thirty percent of these vehicles have significantly lower EF values than guide values on the highway cycle and 10 to 20 percent on the city cycle. Less than ten percent of the vehicles have significantly higher EF values on both cycles. As will be seen throughout there is general concurrence of results both between percents and differences and between all vehicles and passed vehicles. However, as seen in Table 2, there is quite a bit of disagreement between city and highway cycles for the same vehicle. For example, of the 67 vehicles in the less than 90% group on the highway cycle, only 37 of these vehicles are in the less than 90% group for the city cycle. Tables 3 and 4 give the medians and means for the differences and percents by the various factors under investigation. Table 3 includes "all available data while Table 4 presents results for those vehicles which were within the limits set by Federal regulations on HC, CO and NOx emissions. The asterisks (*) indicate the Kruskal-Wallis tests which achieved the nominal 0.01 significance level for testing whether the factor in question has any relationship to the measure in question. For example, in Table 3 the asterisks following "Site" indicate that "site" has a significant effect upon both percents and differences for the city cycle but for neither on the highway cycle. That is, on the city cycle, the city in which a vehicle was operated affected the vehicle's relative in-use/guide fuel economy. These two tables will be summarized by factor headxngs below: All Vehicles: The overall medians and means appear for all vehicles in Table 3 and for all passed vehicles in Table 4. ------- —5— Site: For both groups of vehicles site was a significant factor on the city cycle but not on the highway cycle. It appears that vehicles in Houston had lower EF values for the city cycle relative to the guide than other cities. Model Year: The city cycle showed significant differences for model year for both groups of vehicles. Model year 1975 vehicles had higher EF values relative to the guide than 1976 model year vehicles. There was an insufficient number of model year 1975 vehicles with Highway Fuel Economy Tests to make an adequate comparison on the highway cycle. Model Size; All tests were significant based upon model size. Subcompacts had lower EF values relative to the guide than other vehicles. Cylinders: All tests were significant for number of cylinders. Four cylinder vehicles had lower EF values relative to the guide than other vehicles. On each measure it appears that the vehicles generally are in the ordering four cylinders, six cylinders, eight cylinders from least to greatest. Garb Venturis: Only the test for highway differences on passed vehicles turned out significant. This might be considered a spurious result since the same test on all vehicles which showed the same general trend with a larger sample did not show significance. CID; All tests were significant for engine size as measured in cubic inches displacement. The smallest engine size category, 0-150, showed lower EF values relative to the guide than larger engine size vehicles. Transmission: All tests were significant when comparing manual » transmission vehicles with automatics. Manual transmission vehicles showed lower EF values relative to the guide than automatics. Manufacturer: Manufacturer was a significant factor by all tests. Vehicles in the "Other" category (not AMC, Chrysler, Ford, or GM) had lower EF values relative to the guide than vehicles in the other categories. Catalyst; For both groups of vehicles presence of catalyst only appeared as a significant factor for highway differences. Those vehicles which had no catalyst showed lower EF values relative to guide values for highway differences than vehicles with catalyst. Primary Use; None of the tests showed significance for usual load. ------- -6- Maintained According to Manufacturer's Recommendation?; None of the tests showed significance for this factor. Satisfied with Engine Performance7: This factor turned out significant for both percents and differences on the highway cycle for the group consisting of all avaliable vehicles. Those vehicles for which the owner answered "yes" to the question were in the middle with "no" below and "most of the time" above for EF values relative to guide values. How Often Tuned?: This factor showed significant effect for all vehicles on the city cycle. Vehicles with owners answering that their vehicles were tuned less often than once per year had higher EF values relative to guide values. Last Tune7: This factor turned out significant for both groups of vehicles on the city cycle for both percents and differences. Vehicles owned by people who answered that the last tune was more than a year previous had higher EF values relative to the guide than other vehicles. Note that this result as well as , the above result concerning the frequency of tune could be confounded with the effect of model year. Who Tuned?: Only the percents on the city cycle for the group consisting of all available vehicles showed significant effects from this factor. This should probably be considered non- meaningful significance since there appears to be little range in the medians and means. Mileage Group: Both groups of vehicles showed significance on both tests for the city cycle while none of the tests for the highway cycle were significant. It appears that for the city cycle vehicles with low mileage have low EF values with respect to guide values and as mileage increases EF values approach guide values. No such trend is observed on the highway cycle. Although all four measures (city percents, city differences, highway percents and highway differences) appear to be slightly lower for the group consisting of passed vehicles than for the entire group of vehicles, Kruskal-Wallis tests for differences between the group of vehicles which passed and the group which didn't were far from significant for all four measures. As observed above, the tests on the two groups of vehicles lead to generally consistant results. Tables 5 and 6 present the results of the calculations of the Goodman- Kruskal Gamma and an estimate of the probability of concordance for appropriate groups. These measure the agreement in ranking between Emission Factor test values and Gas Mileage Guide values. The Goodraan- Kruskal Gamma is similar to a correlation coefficient ranging from -1 to +1 and the probability of concordance is the probability that two ------- -7- vehicles drawn at random from the appropriate group would be ranked in the same order by the two determinations of fuel economy. Thetables stratified by vehicle size groups since these groups correspond to the current organization of Gas Mileage Guide and they form a breakdown which would be practical for a consumer considering the purchase of a new vehicle. It is seen that for larger groupings (all vehicles, all 1976 vehicles, etc.) the probability of concordance and the G-K Gamma is larger than for the more discrete breakdowns. This is reflective of the fact that the smaller groups are fairly homogeneous within while a large degree of heterogeneity exists between the groups. These measures are of the same order of magnitude and generally show the same trends when comparing the two groups of vehicles (passed vehicles and all vehicles). Conclusions Based on all avilable data, 68% and 88% of vehicles had Emission Factor fuel economies within 2 miles per gallon of Gas Mileage Guide fuel economies for the highway cycle and city cycle respectively. In terms of relative fuel economies, 67% (highway) and 74% (city) of these vehicles had Emission Factor fuel economies within ten percent of the guide values. Twenty-eight percent on highway and ten percent on city had EF fuel economies more than 2 miles per gallon less than the guide values. Twenty-nine percent and eighteen percent for highway and city respectively had EF fuel economies that were less than ninety percent of guide values. The results for the group of vehicles which were within emission standards were very close to the above. Those vehicles which generally have relatively high fuel economies (e.g., vehicles with engines smaller than 150 CID) showed EF fuel economies lower than other vehicles relative to the guide values. Vehicles which had the least maintenance appeared to have higher EF economies relative to guide values. However, the significance of this is uncertain since the quality of the received maintenance is not known. It could imply that the "maintenance" consisted of carburetor adjustment which was detrimental to fuel economy. On the city cycle vehicles with low mileage have low EF fuel economies relative to guide values and as mileage increases EF values approach guide values. The difference between the total group of vehicles in the analyses and that subset of vehicles which were within emission standards is essentially insignificant. ------- -8- Table 1 Number and Percent of Vehicles in Difference and Percent Groups AH Vehicles Passed Vehicles Highway N7 /> Total Differences < -2 -2 to +2 > +2 Percents < 90 90-110 > 110 231 65 157 9 67 154 10 100 28 68 4 29 67 4 City N % 812 78 712 22 143 601 68 100 10 88 2 18 74 8 Highway N %, 132 41 88 3 40 89 3 100 31 67 2 30 67 3 City N % 385 46 328 11 72 279 34 100 12 85 3 19 72 9 ------- -9- Table 2 Number of Vehicles Cross-Classified by Highway and City Groups Highway All Vehicles Passed Vehicles Differences < -2 -2 to +2 > +2 <-2 37 30 0 -2 to +2 3 153 1 >+2 0 9 0 <-2 , 23 17 0 -2 to +2 2 85 1 >+2 0 3 0 Percents < 90 90 - 110 > 110 < 90 29 90 - 110 > 110 < 90 90 - 110 > 110 24 126 4 0 5 5 22 17 1 14 71 4 0 1 2 ------- -10- Table 3 Medians and Means of Percents and Differences by Factors All Vehicles (* indicates nominal significance for appropriate Kruskal-Wallis test) Highway City N Percents Differences N Percents Differences All Vehicles Site Chicago Denver Houston Los Angeles St. Louis Washington Phoenix Model Year 1975 1976 Model Size Full Size Intermediate Compact Subcompact Truck 231 43 43 43 43 13 44 2 1 230 29 49 51 49 51 Median 94 99 94 93 90 92 96 102 102 94 96 97 95 89 93 Mean 94 97 94 93 90 92 96 102 102 95 * 96 97 97 88 92 Median -1.3 _ o -1.5 -1.4 -2.0 -2.0 -1.0 .7 .5 -1.3 ~~ * / - .6 -1.0 -3.7 -1.4 Mean -1.7 — 9 -1.7 -1.9 -2.5 -2.4 -1.3 .7 .5 -1.7 * - .8 - .7 - .7 -4.1 -1.9 - Median Mean Median 812 208 75 77 78 104 79 191 273 539 150 197 178 196 89 97 * 98 99 93 98 95 96 98 i 100 95 97 99 98 94 99 98 -.5 98 - .3 100 - .1 94 -1.0 101 - .2 95 - .7 97 - .6 98 - .3 103 .1 95 - .7 97 -'.4 99 - .2 98 - .3 94 -1.4 102 - .2 Mean - .5 * - .4 - .2 -1.1 .0 -1.0 - .7 - .4 * .2 - .9 * -'.4 - .1 *' - .4 -1.4 - .1 ------- -11- Table 3 (con't) Highway N Percents Median Cylinders Four Six Eight Carb Venturis One Two Four Fuel Injection CID 0-150 151-250 251-330 331-399 400 Transmission Automatic Manual 55 47 128 52 133 40 6 55 36 47 62 31 175 56 89 93 96 94 92 97 93 89 93 95 98 96 95 88 Mean * 88 93 99 94 93 97 94 * 88 94 95 97 96 * 96 88 Differences Median -3.9 -1.6 - .7 -1.4 -1.6 - .5 -2.6 -3.9 -1.4 -1.2 - .4 - .7 - .9 -3.9 Mean * -4.1 -1.6 - .7 -1.6 -2.0 - .7 -2.0 * -4.1 -1.3 -1.2 - .5 - .8 * - .9 -4.0 N Percents Median 185 133 491 134 496 159 18 187 110 145 245 125 642 170 93 97 98 97 97 97 97 93 97 99 99 96 98 94 Mean * 94 97 99 98 98 97 98 * 94 98 100 100 96 * . 98 95 Differences Median Mean -1.6 - .5 9 » L. - .5 - .5 - .4 - .5 -1.6 - .5 - .1 - .2 - .5 , - .3 -1.1 * -1.5 """ » J - .1 - .5 - .5 - .4 - .4 * -1.5 - .4 \ - .1 .0 - .5 * - ~i -J ™" • J -1.2 ------- -12- Table 3 (con't) Highway Cit} N Percents Differences N Percents Differences Median Mean Median Mean Median Mean Median Mean Manuf ac tur er AMC 5 Chrysler 32 Ford 57 GM 101 Other 36 Catalyst Yes 198 No 33 Primary Use Driver only 145 Driver & 1 48 passenger Driver & 2 28 passengers Maintained According to Mfg. Rec.? * * * * 95 98 -1.0 - .5 22 100 99 - .1 - .2 94 97 -1.2 - .7 108 98 98 - .3 - .4 92 93 -1.6 -1.7 194 96 99 - .6 - .4 97 96 - .7 -1.0 364 98 98 - .3 - .3 88 87 -4.0 -4.5 124 94 94 -1.5 -1.5 * 94 94 -1.1 -1.5 700 97 98 - .4 - .4 91 93 -3.0 -2.7 112 97 97 - .7 - .9 94 93 -1.4 -1.8 518 97 98 - .5 - .5 96 96 - .8 -1.1 186 98 98 - .3 - .5 92 91 -1.5 -2.1 76 95 96 - .7 - .8 i i i • *. j. i ^ —^ -. Yes No Not sure 224 2 4 94 103 94 94 103 95 -1.4 .6 -1.4 -1.7 772 97 ' .6 15 99 -1.6 19 100 97 - .5 110 - .2 97 .0 - .5 1.0 - .8 ------- -13- Table 3 (con't) Highway Cit\ N Percents Median Mean Differences Median Mean N •w- Percents Median Mean _ i — _ - Differences Median Mean Satisfied with' Engine Performance? Yes Most of the Time No 183 31 17 * 93 93 99 91 99 90 -1.5 - .2 -1.8 * -1.8 - .4 -2.4 658 96 58 97 98 95 98 98 95 - .4 - .3 - .8 How often tuned? * Not yet Mfg. Rec 6 Months Year Less often Don't know Last tune? Too new Due, not done 0-6 months 6-12 months Over 1 year Don't know 160 30 21 15 1 4 168 12 45 2 0 4 94 92 93 95 101 99 94 94 93 85 — 99 94 90 93 94 101 96 94 94 92 85 — 96 -1.2 -2.0 -1.4 -1.0 .2 _ 2 -1.2 -1.7 -1.9 -3.4 - .2 -1.5 -2.7 -2.0 -1.5 .2 - .9 -1.5 -1.6 -2.3 -3.4 - .9 400 118 153 106 17 18 397 . 59 v 273 42 19 22 96 97 98 99 100 96 96 98 98 100 101 98 96 98 98 100 109 98 * 96 99 i. i •% 98 „ 99 109 99 - .6 - .4 - .3 - .1 .0 - .5 4 - .6 _ .3 - - .3 i .0 v ., .1 - .3 - .5 - .3 - .9 * - .7 - .5 - .4 - .1 .8 - .4 * - .7 , - .3 - .4 -~* - .3 r .9 - .1 ------- -14- Table 3 (con't) Highway Citi N Percents Median Who tuned? None Dealer Indep. Garage Clinic Self Don't know Mileage Group < 4K 4K-10K 10K-20K 20K-30K > 30K 180 37 4 0 6 4 53 95 66 12 5 94 92 93 — 99 99 92 95 93 97 91 Mean 94 91 93 — 95 96 93 94 94 97 94 Differences Median -1.2 . -2.0 -1.5 - .3 - .2 -1.7 -1.1 -1.5 - .5 -1.9 Mean -1.5 -2.5 -1.8 -2.0 - .9 -2.0 -1.7 -1.7 - .5 -1.7 N Percents Median 451 189 79 15 51 , 27 100 233 289 , 141 49 96 98 98 96 98 99 93 96 98 99 99 Mean * 97 99 99 97 99 104 * 93 96 99 100 100 Differences Median - .6 - .3 - .2 - .5 _ 2 _ 2 -1.0 - .6 - .3 - .1 - .1 Mean -~ - .7 - .4 - .2 - .6 - .3 .3 * -1.3 - .7 - .4 - .1 - .1 ------- -15- Table 4 Medians and Means of Percents and Differences by Factors Passed Vehicles (* indicates nominal significance for appropriate Krusal-Wallis test) Highway \ City N Percents Differences N Percents Differences Median All Vehicles Site Chicago Denver Houston Los Angeles St. Louis Washington Phoenix Model Year 1975 1976 Model Size Full Size Intermediate Compact Subcompact Truck 132 25 25 25 24 6 - 25 2 1 131 18 32 25 35 20 93 96 93 93 90 84 95 102 102 93 96 94 95 87 98 Mean 93 96 93 92 91 85 95 102 102 93 * 95 94 98 87 96 Median -1.5 - .7 -1.8 -1.4 -2.1 -4.5 -1.2 .7 .5 -1.6 - .8 -1.2 -1.1 -4.4 - .4 Mean -1.9 -1.3 -1.8 -2.1 -2.3 -4.6 -1.5 .7 .5 -1.9 * - .8 -1.1 - -5 -4.6 -1.2 Median Mean 385 95 39 40 40 41 41 89 110 275 67 82 - 84 108 42 96 * 97 99 91 97 94 94 98 * 102 95 * 95 97 - t 99 94 99 97 98 100 92 99 94 97 98 103 95 96 97 100 94 101 Median - .6 - .3 _ j^ -1.0 _ o - .8 - .9 - .3 .3 - .8 - .7 - .5 > _ j^ -1.6 - .1 Mean - .6 * - .5 - .1 -1.4 - .3 -1.2 - .7 - .4 * .3 — 9 * - .5 - .4 - .1 -1.5 .1 ------- -16- Table 4 (con't) Highway Cits N Percents Median Mean Cylinders Four Six Eight Garb Venturis One Two Four Fuel Injection CID 0-150 151-250 251-330 331-399 400 Transmission Automatic Manual 36 28 67 27 67 32 6 36 25 12 1 42 17 - 98 34 * 88 93 96 95 91 97 93 * 88 93 94 97 96 ft 95 90 88 94 96 95 91 96 96 88 95 93 96 95 95 88 Differences Median -4.2 -1.5 - .7 -1.1 -2.0 - .6 -2.6 \ 'x -4.2 -1.4 -1.4 - .7 - .7 - .9 -3.8 Mean * -4.4 -1.3 - .8 * -1.0 -2.8 - .7 -2.0 * -4.4 -1.0 -1.6 - .7 - .7 * -1.1 -4.2 N Percents Median Mean P^^^^B Differences Median Mean A A* *» 102 ' 69 213 61 233 79 10 102 60 55 116 52 299 86 94 98 98 98 97 96 95 94 99 100 98 94 97 94 94 98 99 98 98 96 96 * 94 98 101 98 95 * 98 95 -1.7 - .4 - .3 - .4 - .6 - .6 - .8 -1.7 - .2 - .1 """ » J - .8 - .5 -1.2 -1.5 - .4 _ 2 - .5 - .6 - .5 - .7 * -1.5 -*.3 - .1 - .2 - .7 * - .4 -1.3 ------- -17- Table 4 (con't) Highway Citi Manufacturer AMC Chrysler Ford GM Other Catalyst Driver Only Driver & 1 passenger Driver & 2 passengers Maintained According to Mfg. Rec? Yes No Not Sure Satisfied with Engine Performance? Yes Most of the time No N 0 10 36 56 30 80 28 17 127 1 4 106 17 9 Percents Median Mean ft 102 101 92 92 96 96 89 88 93 93 95 96 91 91 93 93 99 99 94 95 93 93 99 98 89 88 Differences Median Mean ft .4 .2 -1.7 -1.8 - .7 - .9 -3.9 -4.4 * -1.5 -2.0 - .8 -1.4 -1.8 -2.3 * -1.6 -1.9 - .3 - .3 -1.4 -1.6 -1.7 -2.0 - .3 - .5 -2.4 -2.9 N 4 20 116 171 74 244 87 39 364 10 10 321 40 24 Percents Median * 97 99 94 98 94 97 96 94 96 99 93 96 97 94 Mean 95 99 97 99 94 98 97 95 97 102 94 97 98 94 Differences Median - .5 - .1 - .8 - .3 -1.6 - -5 - .5 - .9 - .6 - .1 -1.0 - .6 - .4 -1.0 Mean * -1.2 - .3 - .6 - .2 -1.5 - .5 - .6 -1.0 - .6 - .2 -1.3 - .6 - .4 -1.1 ------- -18- N Percents Table 4 (con't) Highway Differences N City Percents Differences Median How often tuned7 Not yet Mfg. Rec. 6 months Year Less often Don't know Last tune7 Too new Due, not done 0-6 months 6-12 months Over 1 year Don1 t kom* Who tuned? None Dealer Indep. Garage Clinic Self Don't know 86 19 14 10 0 3 93 8 27 1 0 3 101 23 1 0 4 3 93 93 92 94 — 100 93 94 92 102 — 100 93 93 83 — 97 100 Mean 94 91 91 94 — 95 94 94 90 102 — 95 94 91 83 — 93 95 Median -1.5 -2.0 -2.0 -1.1 .0 -1.3 -1.7 -2.0 .5 .0 -1.4 -2.0 -5.4 -1.6 .0 Mean -1.7 -2.6 -2.7 -1.6 -1.0 -1.6 -1.7 -3.0 .5 -1.0 -1.6 -2.7 -5.4 -3.0 -1.0 Median Mean 192 64 69 45 6 9 192 34 126 14 7 12 224 89 34 4 22 12 96 97 97 98 111 94 * 95 100 97 100 111 100 96 ' 98 t 97 97 97 100 •i-i ii P 96 96 98 99 109 98 • 95 94 98 97 110 100 96 " 98 99 95 98 100 Median - .7 - .6 - .5 - .3 1.6 _ 7 • / * - .8 .0 - .5 .0 1.3 * .0 - .7 '* * - .3 i - .3 - .4 - .5 .0 Mean ^ - .8 - .7 - .4 — 2 .9 - .3 - .9 .1 - .5 - .6 1.1 .1 -".8 , - .4 - "'. J_ o — » o - .5 .1 ------- -19- N Percents Table 4 (con't) Highway Differences N City Percents Differences Median Mileage Group < 4K 4K-10K 10K-20K > 20K 30 50 40 12 92 93 93 100 Mean 93 93 93 97 Median Mean Median * -1.7 -2.0 58 93 -1.8 -2.0 103 94 -1.7 -2.0 141 97 - .1 - .8 83 99 Mean 93 96 98 99 Median Mean * -1.0 -1.2 - .9 - .8 - .5 - .4 - .1 - .3 ------- -20- Table 5 Goodman-Kruskal Gamma and the Probability of Concordance All Vehicles Model Year both 1 Q7*; j-y 1 J ti tt ii n it Model Size All A1 1 riJLJL Fullsize Intermediate Compact Subcompact Truck HIGHWAY n gamma p n 231 .7638 .8819 812 "I — - 97^ J. — — e.1 J 65 56 48 65 38 CITY gamma .8172 ~IT\r\ • / / JU .7328 .5394 .5875 .6962 .3239 £ .9086 OQfiC . OO\)J .8664 .7697 .7938 .8481 .6620 1976 All 230 .7632 .8816 539 .8585 .9293 Fullsize 29 .5700 .7850 85 .6873 .8437 " Intermediate 49 .4513 .7065 141 .6742 .8371 " Compact 50 .7333 .8667 130 .6889 .8444 " Subcompact 49 .6927 .8464 131 .7219 .8610 11 Truck 51 .5389 .7695 51 .8010 .9005 ------- -21- Table 6 Goodman-Kruskal Gamma and the Probability of Concordance Passed Vehicles Model Year both 1Q71! j~y / J n ii n ii n 1976 n ii ii n n Model Size All All jTl-LJL Fullsize Intermediate Compact .. Subcompact Truck All Fullsize Intermediate Compact Subcompact Truck n 132 i JL 131 18 32 25 35 * 20 HIGHWAY gamma .7853 \ .7847 .5046 .5253 .6637 .7036 .3467 £ .8765 .8924 .7523 .7627 .8319 .8518 .6734 n 385 19 10 21 37 22 275 48 72 63 71 20 CITY gamma .8156 777Q • / / / y .7458 .2500 .6647 .7539 .1034 .8699 .6758 .7709 .6446 .6870 .6508 £ .9078 QOQfl * OO.7W .8729 .6250 .8324 .8770 .5517 .9350 .8379 .8855 .8223 .8435 .8254 ------- -22- Abstract Data from 842 vehicles (model years 1975-1976) in the Fiscal Year 1975 Emission Factor Program are utilized to examine the differences between fuel economies derived from EPA tests on in-use, consumer-owned vehicles and the appropriate values for each vehicle which appear in the Gas Mileage Guides. The discrepancies are examined in terms of absolute differences and percentages. Various vehicle classification, maintenance and utilization factors are investigated to determine their relationship to these discrepancies. The agreement in ranking of vehicles on fuel economy between EF and guide determined economies is also investigated. All analyses are performed on all available vehicles and on the subset of vehicles which pass national standards on HC, CO and NOx emissions. ------- |