Why MPG is a dumb unit for fuel economy

Miles per gallon—mpg—is the default measure of gas mileage in the United States, but it may not be the best one, argues “Engineering Explained” host Jason Fenske. Here’s why he’d rather see a different unit of measurement for fuel economy.

Fenske doesn’t have a problem with the individual units—the mile and the gallon—but says something “weird” happens when you put them together. He explains that with a question: Is it better to double the fuel economy of nine cars from 50 mpg to 100 mpg, or double the fuel economy of just one car from 5 mpg to 10 mpg?

When just looking at efficiency, increasing the fuel economy of the nine 50-mpg cars is the best way to go. But once you factor in distance driven, it’s the other way around.

Gas pump

Gas pump

That’s because mpg is a measure of distance (miles) per volume (gallon). Fenske notes at this point that most other countries flip this around, measuring fuel economy in volume per distance. In Europe, for example, the default unit is liters per 100 kilometers. Measuring fuel economy in gallons per mile would be more straightforward, he says.

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U.S. Corporate Average Fuel Economy (CAFE) targets are already calculated in a manner that’s closer to gallons per 100 miles, Fenske notes. They’re then converted back into mpg to make the numbers easier for the general public to understand, a process called a “harmonic mean.” Window stickers even show fuel economy in gallons per 100 miles in smaller print under the mpg figure.

Increasing the mpg of two cars by the same amount also doesn’t mean they save the same amount of fuel. A gain of 1 mpg will equate to more fuel savings in a car that started out getting 5 mpg than a car that started out getting 50 mpg. Watch the full video to see the math supporting this, and if you want more efficiency, check out this video explaining how big tire sidewalls are your friend.

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