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Why MPG Isn't the Best Metric

Americans have been schooled to think of fuel efficiency in terms of miles per gallon. That’s what carmakers advertise. That’s what drivers expect. But it’s not necessarily the best metric.

That was the conclusion of the team behind GM’s forthcoming Chevy Volt, which threw out the notion of mpg as a way to come up with an alternative vehicle. Others have also advocated eliminating mpg as the default point of reference for a car’s efficiency. Duke University’s Richard Larrick, for instance, argues that the better question to ask is how much gas is being burnt at what cost, to get from point A to point B. His preferred metric: gallons per mile.

Take, for example, a Ford Focus, one of the most popular models bought during the cash-for-clunker bonanza, and the Ford F-150, one of the most popular trade-ins. The Focus gets 28 miles per gallon, the F-150 only 11. That tells us something — that the little Focus is more efficient than the massive F-150. But it tells us nothing about how much it costs to use the vehicles or how much we are burning to get to where we are going; at least not without some calculations.

That is where gallons per mile comes in. Say you want to drive 100 miles, from the outskirts of New York to downtown Philadelphia. In the Focus, that requires 3.5 gallons and less than $10 (at a price of $2.75 per gallon). For the least efficient F-150, it’s 9 gallons and $25. So if you take the Focus, you can buy road munchies for all and still end up spending less than if you drove the F-150. (To check out your car, go to this gpm calculator.) That may sound obvious, but people don’t generally think that way; the point is that gallons per mile provides a more direct way of monetizing the cost of your travel.

GPM also provides a more cogent way to think about emissions. Once you know how much gas you are using, or saving, it’s a simple calculation: A gallon of gas burned accounts for about 20 pounds of CO2; a ton of carbon is emitted for every 100 gallons burned. So in the trip above, driving the Focus meant 110 pounds less of CO2.

Finally, relying on mpg coaxes even green-oriented car buyers into poor decisions, because the metric is confusing. Consider the following example: Driver A has traded in her 35-mpg car for one that gets 50 mpg. Driver B has upgraded the mileage of his car from 15 to 25 miles per gallon. Who is saving more gas?

Obviously, Driver A, whose mpg increased more. Obvious, and wrong.

Do the math. At 15 mpg, Driver B consumed 66.6 gallons to go 1,000 miles; at 25 mpg, he used 40 gallons. Driver B saved more than 26 gallons. Driver A consumed 26.7 gallons in her 35-mpg car to go 1,000 miles, and 20 gallons in her super-efficient 50-mpg car. She saved less than 7 gallons. How can this be? Basic math tells us the percentage increases faster starting from the lower base — an important point that mpg totally obscures.

Miles per gallon is the metric we’re used to, and there is some virtue in familiarity. But it would be useful for dealers to add the GPM number to their stickers — and thinking drivers should certainly add the idea to their mental tool kit.

Where does the Chevy Volt fit in to all this? Well, theoretically, it could get infinite miles per gallon; if you kept plugging it back in before going 40 miles, you would never need to stop at a gas station. If you use GM’s guesstimate, using a draft version of methodology being devised by the EPA, of 230 miles per gallon under city conditions, that would convert to less than five gallons per 1,000 miles — a quarter of the amount it takes to run the newest Toyota Prius hybrid, the current gallons-per-mile champ.

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