Why MPG May Be Outdated

With word of the plug-in hybrid Chevy Volt’s claimed 230 mpg and the all-electric Nissan Leaf’s alleged 367 mpg, it’s become clear that the way we look at mileage has to change. Consumers have to be able to compare cars apples-to-apples, not apples-to-potted-plants, and what they’re most interested in is how much it will cost to drive a specific car.

GM offered a glimpse into what might be the right way to measure that when it indicated in a press release that the Volt would cost roughly 2.75 cents per mile to operate under the best circumstances, based on a national average cost for electricity.

Using some simple math, and assuming that gas averages $2.75 a gallon nationally, the Prius, which gets 50 combined mpg, would cost about 5.5 cents per mile ($2.75 divided by 50 = 5.5 cents per mile). What that tells me is that the Volt’s mileage is actually about twice as good as the Prius’, at least as it pertains to my wallet. Going backward from that math, as far as consumers are concerned the Volt really gets 100 mpg.

I don’t know yet what the Leaf will really cost per mile, but using the figure Nissan has announced, you could assume that it costs 1.72 cents per mile, or roughly a third the cost of a Prius. That’s about 160 mpg, practically speaking.

None of this is meant as a disparagement of the value of cars that get such high mileage; it just means that the method automakers are using definitely makes it seem as if outlandish mpg numbers need to be revised to make them more coherent to average consumers. Of course, as the price of gas rises (or falls), those gains become more (or less) impressive.