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Kevin Bullis

A View from Kevin Bullis

Miles-Per-Gallon Math

When a 2-miles-per-gallon improvement is better than improving by 16 miles per gallon.

  • June 20, 2008

Say you’ve got two cars in your garage. One of them gets 34 miles per gallon; the other gets only 12. You drive both cars 10,000 miles in the course of a year.

Would you save more gas by a) trading in the 34-miles-per-gallon car for one that gets 50 miles per gallon, or by b) trading in the 12-miles-per-gallon car for one that gets 14 miles per gallon?

New experiments suggest that people tend to pick a). After all, a 16-miles-per-gallon improvement seems better than an improvement of just 2 miles per gallon.

The right answer is b).

If you start driving the 50-miles-per-gallon car instead of the 34-miles-per-gallon car, you’ll save 94.1 gallons of gas per year.

If you start driving the 14-miles-per-gallon car instead of the 12-miles-per-gallon car, you’ll save 119 gallons per year.

The math is simple arithmetic. Divide the total number of miles driven (10,000) by the miles per gallon to get the total gallons used to drive that distance. For 12 miles per gallon, the answer is 833. For 14 miles per gallon, it’s 714.

The fact that people guess a) rather than b) suggests that miles per gallon isn’t a useful metric for describing a vehicle’s gas consumption, say the researchers who did the recent experiments. A much more direct way to measure fuel consumption is an estimate of the amount of gas required to travel a given distance.

Such a number would also make it easier to convey just how much could be saved by moving closer to work or taking public transportation. And it renders the difference between a 12-miles-per-gallon SUV and a 50-miles-per-gallon hybrid more impressive, making it clear just how much fuel gas guzzlers are using. It takes 833 gallons to travel 10,000 miles in the former vehicle; it only takes 200 gallons to go 10,000 miles in the latter.

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