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Real MPG - Putting the Truth in Your Tank: Methodology

July 13, 2006

Environmental Working Group calculated the amount of gasoline and oil saved in 2005 if automakers had met federal fuel efficiency or miles per gallon (MPG) standards based on real world driving conditions beginning in 1978. In order to make this calculation it was necessary to know the composition of the U.S. car and truck fleet, how many of each model year were on the road in 2005. EWG used data from a commercial automobile information company to determine how many cars and light trucks from each model year were on the road as of January 2006. We estimated how many miles each model year is currently driving using data from the U.S. Department of Energy on miles driven by age of car (DoE 2001). The DoE reported, for example, that cars less than one year old traveled an average of 14,500 miles per year while cars ten years old and older traveled an average of 8,100 miles per year.

EWG then referred to Department of Transportation data which shows the automakers' reported miles per gallon for cars and light trucks using the government's inaccurate driving test (DoT Fuel Efficiency 2006). We used a recent Consumer Reports analysis of city and highway fuel efficiency for more than 300 cars and light trucks to estimate the ratio of city to highway miles per gallon (CU 2005). We then applied this city/highway fuel efficiency ratio to MPG data from the DOT -- data that are derived from the current, inaccurate driving test.

We then discounted the highway figure by 22 percent and the city figure by 10 percent — the same percentages by which the EPA discounts highway and city miles per gallon figures before placing them on cars' window stickers. We discounted the highway figure by an additional 10 percent and the city figure by an additional 15 percent because the EPA has estimated that a real world driving test would further reduce the window sticker figures by these percentages (EPA Sticker 2006). Finally, we combined the highway and city figures, using EPA's estimate that 45 percent of driving is city driving and 55 percent of driving is highway driving (FR 2006). We multiplied our final figure (.73) by the DoT figures for each model year to determine the real world miles per gallon figures for each model year.

Then we divided the total miles traveled per year by cars and light trucks in each model year by the real world miles per gallon figure for each model year. The result was the gallons of gasoline currently consumed. EWG's calculation is almost exactly the same as DoT's most recent published estimate of car and light truck gasoline consumption (DoT VMT 2004). We made the same calculation using the miles per gallon standards that automakers are required to meet under federal law. The result was the gallons of gasoline that cars and light trucks should be consuming. Then we subtracted the gallons cars should be consuming from the gallons that cars are currently consuming. The difference is more than 33 billion gallons, the total number of gallons of gas saved in 2005 if cars had to meet federal mileage standards based on real world driving tests. The result includes cars from the 1978 model year and later and light trucks from the 1982 model year and later. Cars were first required to meet federal mileage standards in 1978 while light trucks received their own standards beginning in 1982.

The U.S. Department of Energy reports that the heat content of each barrel of gasoline is 5.218 million British Thermal Units (BTUs). There are 42 gallons in each barrel, so 33 billion gallons of gasoline is equivalent to about 789 million barrels or about 4.1 quadrillion BTUs. The DoE reports that there are 5.8 million BTUs in each barrel of oil so we divided 4.1 quadrillion by 5.8 million to determine that the U.S. would be saving about 710 million barrels of oil per year if automakers met federal mileage standards based on a real world driving test.