The MPG illusion studies
Study 1: In the first study, college students were asked to rank each of the following vehicle changes (old car vs. new car) in terms of total gas saved, assuming that all the vehicles were driven 10,000 miles (shown in a random order):
A) 34 to 50 MPG
B) 18 to 28 MPG
C) 42 to 46 MPG
D) 16 to 20 MPG
E) 20 to 22 MPG
The majority of participants ranked the changes in order of the linear increase in improvement in mpg, (16 mpg for A, 10 mpg for B, etc.) However, in reality, B and D save more gas than A; D and E save more gas than C. Only 1 participant in 77 gave the correct order in terms of gas saved per 10,000 miles: B, D, A, E, C.
The A vs. B comparison is close to a family decision we made (to replace a minivan that got 18 mpg with a small station wagon, or to replace an efficient sedan with a hybrid compact). We were shocked to discover that option B saves twice as much gas as does A. Over 10,000 miles, B saves 198 gallons; A saves 94 gallons.
Study 2: A second study asked college students to price the gas savings from adding more efficient engines to a car that gets 15 mpg and costs $20,000, where the only feature that varies across vehicles was the mpg. Linear reasoning led them to undervalue improvements to 19 and 25 mpg and overvalue improvements to 55 mpg (under a range of discount rate assumptions).
Study 3: A third study showed that the mpg illusion could be broken by expressing efficiency as gallons per 100 miles. In this study, we asked a cross-section of adults to think about a town’s fleet of vehicles that all drove the same distance in a year. Half the respondents got 15 mpg and half got 34 mpg. They were asked to choose between replacing the low efficiency cars with vehicles that get 19 mpg, or the high efficiency cars with vehicles that get 44 mpg. Three-quarters preferred the second option when expressed as mpr. However, when gallons per 100 miles information was also given, 64 percent correctly preferred the first option, which saves about 1.4 gallons for every vehicle replaced (compared to roughly .7 when replacing the 34 mpg vehicles).