Miles per gallon, liters per 100 kilometers

I bumped across a 2008 New York Times blog post called “The Illusion of Miles Per Gallon,” which started out with a quiz:

Which of the following would save more fuel?

a) Replacing a compact car that gets 34 miles a gallon with a hybrid that gets 54 m.p.g.

b) Replacing an S.U.V. that gets 18 m.p.g. with a sedan that gets 28 m.p.g.

c) Both changes save the same amount of fuel.

The post quotes two management professors from Duke University who were pointing out that the US method of quantifying fuel consumption works against human intuition. We look at those numbers in the quiz and assume that since the difference between 34 and 54 is 20, while the difference between 18 and 28 is only 10, that the correct answer ought to be (a). Heck, it’s obvious that (a) is exactly twice as much fuel savings as (b)!

Except that the correct answer is, of course, (b), which in fact is nearly twice as much savings as (a). Miles per gallon is a really confusing mathematical expression.

The Duke professors suggest that instead of using miles per gallon, the metric should be gallons per mile, expressed as gallons per 100 miles. Under that metric, we get the following numbers:

18 mpg = 5.56 gallons per 100 miles
28 mpg = 3.57 gallons per 100 miles
34 mpg = 2.94 gallons per 100 miles
54 mpg = 1.85 gallons per 100 miles

Now the difference between the four rates of fuel consumption becomes much more obvious, nor is a calculator required to figure it out.

This is how the Europeans do it — in liters per 100 kilometers. It drove me nuts when I first got here because what I wanted to know was, how many miles per gallon is 6.7 liters per 100 kilometers?? (35 mpg.) I spent my whole driving life using one metric; how was I supposed to be able to relate this new one to the old one?

Well, I can’t, not without a converter (which I was pleased to find is built right into the Google search box). These days I’m used to the l/100km thing, and appreciate it much more. Certainly it makes it far easier to compare the efficiency of various cars, which made looking for a new car a whole different experience.

One thing is the same, though: car manufacturers still exaggerate the bejeebers out of their fuel efficiency estimates, no matter which metric is used.


About Fletcher DeLancey

Socialist heathen and Mac-using author of the Chronicles of Alsea, who enjoys pondering science, politics, well-honed satire (though sarcastic humor can work, too) and all things geeky.
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2 Responses to Miles per gallon, liters per 100 kilometers

  1. Pit says:

    Quite interesting. For me, having lived in Germany for the first 55 years of my life, the difficulty is to get used to miles per gallon. I didn’t know about the Google searcbox, so I created my own conversion table in a spread sheet.
    Best regards from southern Texas,

  2. Lilaine says:

    Talk about it!
    I’ve had a hard time with the trip cost comparison I posted here a while ago. There was not only mpg-l/100km conversion, but the $-€ one… lost a couple of neurons in the deal… conversion costs, probably. 😉

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