The problem with using miles per gallon to measure fuel economy

One expert argues we should be measuring gallons used per 100 miles instead

By Collin Woodard / Jalopnik3 min readUpdated August 6, 2024

For regular people, fuel economy is one of the most important factors when it comes to deciding what new car to buy, right up there with price, reliability and whether or not Apple $AAPL CarPlay is available. And it makes sense. If one car gets 20 mpg and the other gets 25 mpg, fueling the latter is going to cost less in the long run. It’s a simple, straightforward number that makes it easy to figure out which car uses less gas. Except, when you dig a little deeper, measuring fuel economy in miles per gallon isn’t ideal, Vox recently argued.

To better understand where that opinion is coming from, let’s take a look at how the article opens:

Time for a pop quiz. Which of these trades saves more gas:

A) Swapping a car that gets 25 miles per gallon (MPG) for one that gets 50 MPG, or

B) Replacing a car that gets 10 MPG with one that gets 15 MPG.

If you said that A conserves more gas, you’re mistaken. And it’s not even close.

Here’s why: In the first scenario, the old vehicle getting 25 MPG uses four gallons of gas to travel 100 miles, while the new one at 50 MPG uses two. In the second scenario, the vehicle getting 10 MPG needs 10 gallons to traverse those 100 miles, while the one at 15 MPG uses 6.7, saving 3.3 gallons — fully 65 percent more than in scenario A.

If you answered wrong, don’t be too hard on yourself. You’ve succumbed to the MPG Illusion, a widespread fallacy that can easily distort perceptions of a car’s efficiency and muddle debates about transportation and climate policy.

The takeaway here isn’t necessarily that miles-per-gallon figures are incorrect. If you buy a car that gets 50 mpg, you’ll spend less money on gas than one that gets 25 mpg. It’s more that we tend to underestimate the benefits of squeezing a few extra miles per gallon in a car that gets terrible gas mileage versus making an already fuel-efficient car even more efficient. If the only people who actually used the EPA’s mileage ratings were car buyers, that wouldn’t be a problem, but it’s also an important factor when it comes to setting policy, which is a much bigger deal.

Federal Corporate Average Fuel Economy standards are calculated in miles per gallon, but it also sets different requirements for cars than it does for crossovers and pickup trucks. The Gas Guzzler Tax also lets truck and SUV drivers off the hook even though, at this point, four out of five vehicles sold in the U.S. are trucks and SUVs. Focusing on MPG also incentivizes automakers to make already efficient vehicles more efficient instead of finding ways to get a 16-mpg Suburban up into the 20s.

Instead, Vox argues the U.S. should instead focus on gallons per 100 miles traveled or GPHM. Europe already does essentially the same thing, measuring fuel economy in liters per 100 kilometers. “They do it that way because fuel consumed per mile is directly related to energy use and directly related to emissions, whereas our MPG is not,” Kate Whitefoot, an associate professor of engineering and public policy at Carnegie Mellon, told Vox.

While it would likely take a little getting used to, calculating GPHM wouldn’t be anything new. Since 2013, it’s been listed on a new car’s sticker along with an estimated annual cost for fuel. Those figures just aren’t given as much attention as MPG. Switching the focus to GPHM would still make it easy for buyers to understand how much gas their car will actually use, but it would also likely do a better job of encouraging them to avoid gas guzzlers.

There’s a lot more in the original post, so head on over to Vox and give the whole thing a read.

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