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Using gallons per mile to measure fuel efficiency

Andrew Zumwalt, associate state specialist, Personal Financial Planning, University of Missouri Extension


Miles per gallon (MPG) may not be providing you with the best measure of the fuel efficiency of your car — especially when you are buying a new car and trading in your old one. If you are looking to compare the efficiency of your old car with a new one, consider using gallons of gasoline used per mile.


In a recent issue of Science magazine, consumers ranked the most efficient replacement strategy for their cars. In the study, about 60 percent of people rated replacing a 34 MPG car with a 50 MPG more efficient than replacing a 18 MPG car with a 28 MPG car. People also rated the pair of cars with the highest increase in MPG as the most fuel efficient, and the pair with the lowest increase in MPG as least efficient.


However, increases in MPG can be misleading. In the example above, replacing the 18 MPG with the 28 MPG car actually saves more gasoline than replacing a 34 MPG car with one that gets 50 MPG. The table below helps illustrate:


Table 1: Car comparison
Original car Gallons of gasoline consumed per 10,000 miles New car Gallons of gasoline consumed per 10,000 miles Gallons of gasoline saved
34 MPG 294 50 MPG 200 94
18 MPG 556 28 MPG 357 199


Instead of comparing miles per gallon, the measurement used is gallons of gasoline per 10,000 miles. Replacing the least efficient car (18 MPG) with one slightly more efficient (28 MPG) saved 199 gallons of gasoline per 10,000 miles. Only 94 gallons of gas was saved by replacing the more efficient car (34 MPG) with one that is most efficient (50 MPG). MPG is misleading because the relationship between miles and gallons is not straight. As the MPG of a car increases, the amount of gallons for gasoline used decreases, but the savings gets smaller with each unit increase in MPG. Table 2 helps illustrate the point:


Table 2: Savings in gasoline from 10 MPG increments
Car MPG Gallons of gasoline consumed per 10,000 miles
1 10 1,000
2 20 500
3 30 333
4 40 250
5 50 200
6 60 166


In Table 2, the 10 MPG increase between car 1 and car 2 saves 500 gallons of gasoline. The 10 MPG increase between car 5 and car 6 only results in a savings of 34 gallons.


When you are shopping for a car, be sure to compare cars by the gallons of fuel consumed over 10,000 miles instead of miles per gallon. Replacing an average fuel efficient car with a very efficient car may not save you as much as replacing a very inefficient car with a car that is slightly more efficient.


Larrick, R. P. & Soll, J. B. 2008. The MPG illusion. Science 320:1593-1594.



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