Here in the US, we’re so used to measuring a car’s efficiency in “miles per gallon” that it seems impossible to think that there might be a better way.

Canadian Energuide labelBut there is, and it’s used just north of the border. In addition to miles per gallon, Canada’s  fuel efficiency labels use a far more revealing and intuitive measure: “litres per 100 kilometres.” The important thing isn’t the switch to the metric system, it’s that the division is flipped on its head.  The distinction between “miles per gallon” and “gallons per mile” might seem trivial, but it makes all the difference between confusion and enlightenment.

Here’s an illustration.  Imagine that your car gets just 10 mpg, and you upgrade to a car that gets 20 mpg. And at the same time, imagine that your neighbor boosts her car from 40 mpg to 50 mpg.  In both cases, it’s a 10 mpg improvement—which would make you think that the two shifts have done the same amount of good.

Not so!

Turn the math on its head, and you’ll see why. The 10 mpg car burned 10 gallons every 100 miles.  The new 20 mpg car burns 5 gallons every 100 miles. So the upgrade from 10 mpg to 20 mpg saved 5 gallons of gas for every 100 miles of driving. But your neighbor shaved her gas consumption from 2.5 gallons per 100 miles to 2 gallons per 100 miles—saving just half a gallon of gas every 100 miles.

In short, the upgrade from 10 mpg to 20 mpg was 10 times as important as the upgrade from 40 mpg to 50 mpg. And yet, both upgrades involved an increase of 10 mpg.

  • For more examples of mpg math, I’d encourage you to play with this  great new fuel efficiency calculator from David Kroodsma at  climatecentral.org.

    As we’ve written before, the miles-per-gallon view of fuel efficiency can focus our attention on the wrong things. People who care about reducing oil consumption naturally focus on the cars with really high mpg ratings. A car that gets 100 mpg seems so much better than a car that gets just 50 mpg! But for oil consumption, the biggest bang-for-the-buck is at the low end. Shifting one vehicle from 15 mpg to 20 mpg can save nearly as much fuel as getting a Prius driver behind the wheel of an all-electric Leaf!!  But in order to see the importance of boosting fuel efficiency at the low end, Americans have to do some arithmetic—while Canadians can see the difference at a glance.

    New EPA fuel economy label

    Luckily, EPA itself is taking steps to deal with the flaws in mpg math.  They’ve just announced a new fuel economy label that focuses on how much money people spend on gas—which is the same thing, basically, as how much gas they consume for a given amount of driving. And in an especially clever move, the label tells you how much money you can save (or lose!) over five years by choosing a particular car, compared with the average car or truck sold. In short, the new label turns fuel efficiency into a pocketbook issue—and one that’s immediately apparent, just when car shoppers are already focused on a car’s sticker price.  Smart move!