MPG is specious

I read something today that blew my mind:

If you raise a guzzler’s fuel efficiency from 15 miles per gallon to 35 miles per gallon, you save almost four gallons per 100 miles. But boost a fairly efficient car from 35 mpg to 100 mpg, and you save less than two gallons in the same distance.

Of course by “blew my mind” I mean, I totally didn’t believe it and stopped everything I was doing and started running the numbers. And that’s when my mind was blown. Do people know this? That the gas savings of a 40mpg engine over a 30mpg engine is not the same as 30 over 20? I doubt it.

How do you convert miles per gallon into gallons per mile? Ugh, math! Turns out it’s easy. Just take the reciprocal of mpg. For example 20mpg = 1/20gal/100mi = 0.05gal/100mi. The problem there is that really small fractions are not very illustrative. So it’s better to think in terms of gallons per X miles, such as 100. This, as it happens, is how fuel economy is listed in Europe, in liters per 100 kilometers (l/100km).

So a 20mpg vehicle would translate to 5gal/100mi. 30mpg = 3.3gal/100mi for a gas savings of 1.7 gallons (per 100 miles). Here’s where it gets crazy. 40mpg = 2.5gal/100mi for a gas savings of only 0.8 gallons over a 30mpg vehicle. What seemed like a constant improvement, 20mpg to 30 and then 30mpg to 40 actually represents half as much improvement, and it only gets “worse.” Every 10mpg improvement actually represents an increasingly diminishing improvement in actual fuel economy. Who knew?

Chart showing the relationship between gallons per 100 miles and miles per gallon

As the article above mentions, what this means is that more effort, attention, and frankly laws should be directed at getting the gas guzzlers off the road (or up to 30 or 40mpg), rather than focusing on 100mpg supercars. In any case, I’ll be riding my Vespa to work, sipping 1.8 gallons per 100 miles (55mpg).

1 Comment

You’re absolutely right, the reciprocal math obscures the importance of focusing on older cars, and doesn’t help people make informed decisions about which car to purchase.

Generally any measure of efficiency is denoted in output/input, e.g., amount of grain grown per acre. That’s because you tend to think of the input (land) as constant and you want to know how much more grain you can squeeze out of your acreage, but for fuel it’s backwards. You assume mileage (output) remains fairly constant but fuel (input) is being conserved.

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