Pocketmint

small change toward a rich life
24
June
2008

Gas mileage: we’re doing it wrong

Gasoline prices are on the mind of nearly every American right now. Here in Seattle, where gas is currently climbing past the $4.50 mark, we like our hybrid cars. A lot.

But Seattle had an environmentally ‘green’ culture long before the recent spike in gas prices, which suggests that a lot of those hybrids are being purchased by people who already had reasonably fuel-efficient cars.

Which is fine, but upgrading your existing 30 mpg vehicle to a 50 mpg hybrid won’t help nearly as much as upgrading from 15 mpg to 25 mpg.

Wait, what? Shouldn’t an increase of twenty miles per gallon be twice as good as an increase of ten miles per gallon?

Actually, no — in this case, it’s only half as good.

Turns out that our intuitive math is all wrong, thanks to the ‘miles per gallon illusion’. As explained last week on NPR’s All Things Considered, the number that really matters is ‘gallons per mile’.

Like this: a 15 mpg minivan uses .0667 (1/15) gallons per mile — or to make the numbers a bit easier, 6.67 gallons per 100 miles. A 25 mpg station wagon needs 4 gallons to go 100 miles. Trade the minivan for the wagon, and you’re saving 2.67 gallons per 100 miles. (That’s worth about $12 in Seattle right now.)

Now, a 30mpg sedan uses 3.33 gallons per 100 miles, compared to 2 gallons per 100 miles from a 50mpg hybrid. That’s a savings of 1.33 gallons per 100 miles (currently $6).

Or, only half the improvement of the 15-to-25 mpg upgrade.

The way cars are advertised in the States (the rest of the world, apparently, gets it right) leads people to make the wrong decisions about which cars to upgrade. A 3 mpg improvement hardly seems worth the bother … and if you’re already getting 30 mpg, it’s likely not. If you’re going from 12 mpg to 15 mpg, though, the difference is a lot bigger than you probably think.

I confess that I am periodically tempted to trade our 1999 Civic (rated as a respectable 24 mpg in-city, but in practice we get over 30 mpg) for a shiny new (or at least new-ish) hybrid. But even though we could literally cut our gas bill in half, it wouldn’t come close to making up for the roughly $16K more we’d have to spend to trade up to, say, a 2006 Prius. Gas will need to be a lot more expensive for that math to work out.

Update 14-Jul-08: Rick Larrick of Duke University offers a more thorough mathematical explanation of the problem.

(Photo by Casey Hamilton.)

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2 responses

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  1. oneaustin says

    You should post a formula for us to calculate the gallons per mile.

    • Karawynn says

      Okay! If you assume X is the number of miles per gallon, then:

      1/X * 100 = gallons per 100 miles.

      (Gallons per mile is always a decimal, so the standard is to use gallons per 100 miles.)

      There are a couple of nice calculators here, for comparison purposes: http://www.gpmcalculator.com/



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