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Is Miles Per Gallon Everything?

By Andrew Zumwalt

Truth can get lost in hype. The media has recently reported energy analyst price predictions of $4-$5 per gallon of gasoline for this summer. With such a high price for gasoline, consumer’s thoughts again start turning toward the efficiency of their vehicles. Higher MPGs provide better fuel economy, but is Miles Per Gallon the only consideration?

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 (or per 1,000 or 10,000 miles).

In a past 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

1

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.