BUYER’S GUIDE: How the EPA Changes Its Fuel Economy Calculations

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If fuel mileage ratings on vehicles you are shopping for seem a bit low, it is no accident.

Changes that the EPA made to how fuel economy is calculated and displayed on Monroney (new car) stickers and in advertisements have begun to show up. We first noticed the change when we saw that the 2017 Mazda6 had a lower fuel economy rating than the 2016 model, which had the same drivetrain. The Mazda6 dropped by one mpg.


The next clue was the official rating of the new 2017 Chevy Colorado and GMC Canyon. Unlike the unchanged Mazda6, these popular trucks are outfitted with a new V6 engine that has fuel saving technologies its prior model year did not. Chevy also added a new eight-speed transmission, with two more gears than the 2016.

General Motors clearly expected the MPG of the Colorado and Canyon would go up. GM’s headline announcing the new fuel-saving technologies was “Colorado Resets the Bar For Midsize Segment.” In fact, the combined EPA fuel mileage of the Colorado and Canyon went down by one mpg, which now puts the GM trucks below the equivalent Toyota Tacoma’s combined rating.


The EPA has a long history of adjusting fuel economy ratings. There was a time when the printed mileage on a window sticker was an open joke. No consumer took the exaggerated ratings seriously in the 1970s and 1980s. However, after a series of adjustments, the EPA seemed to get it right. Having tested cars each week for three years, it is obvious to this tester that vehicles now meet or exceed their estimated mileage ratings. Most automotive testers report higher mileage numbers than automakers and the EPA claim.

However, a vocal minority of drivers frustrated that they could not achieve mileage results that extensive testing and other drivers could seems to have prompted the EPA to act. A skeptic might consider that a lower MPG rating aids the EPA in its efforts to reduce emissions and improve mileage, by forcing automakers to work even harder to get to the milestones set forth by the agency.

In its announcement of the revised ratings, the EPA said: “The new calculations are based on test data from model year 2011–2016 vehicles. So, they better reflect today’s vehicle fleet of more fuel-efficient vehicles and advanced technologies such as hybrids and turbocharged engines. Most vehicles are not affected by the new calculations. Some fuel economy estimates will decrease by one mpg, and a small number may be two mpg lower.”


To make it easy for consumers to compare a prior model year vehicle to a 2017 model, the EPA has set up a comparison page. To see how many of the most popular models compared to the old ratings we tested three industry leaders in addition to the Mazda6 and Colorado and Canyon. The top-selling Honda Accord has a combined EPA one-mpg reduction.

The Toyota RAV4 kept its 25 mpg combined rating, but its highway rating dropped by one mpg. The Toyota Corolla also dropped by one mpg combined. We selected this sample based on sales, and models we knew to have the same drivetrain as the prior model year. In every case, the EPA reduced the mileage rating.

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