The steady rise of corporate average fuel economy rules through 2025 puts increasing pressure on manufacturers to boost the gas mileage of all vehicles.

Lately, carmakers are challenged by cheap gas and a resulting public preference for utility vehicles of all sizes against sedans and hatchbacks.

Now, a slight tweak to EPA calculations for the widely-used EPA fuel-economy ratings poses a further challenge.

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For the 2017 model year, the EPA has slightly changed the calculations for its gas-mileage ratings. (Full details of the changes, first announced in February 2015, are here.)

The updates apply to what's called the "coastdown" part of the cycle, in which carmakers calculate the mechanical resistance and aerodynamic drag of the car at higher speeds.

The new calculations mean that a number of vehicles will see their ratings fall slightly, generally in the highway part of the three ratings quoted: city, highway, and combined.

EPA fuel economy ratings for 2017 vs 2016 vehicles, showing effect of changes [graphic: EcoModder]

EPA fuel economy ratings for 2017 vs 2016 vehicles, showing effect of changes [graphic: EcoModder]

As a post on the EcoModder gas-mileage forum indicates, the results vary.

The EPA rating for the V-6 version of the 2017 Toyota Camry mid-size sedan, for instance, loses 1 mpg highway over the identical 2016 car, from 31 to 30 mpg.

That's enough to cut its overall figure from 25 mpg to 24 mpg combined.

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The 2017 Kia Rio Eco, on the other hand, also loses 1 mpg in its highway rating—from 37 to 36 mpg—over the 2016 model, but its 31 mpg combined rating stays the same.

The same applies to the 29-mpg combined ratings for the 2016 and 2017 Acura ILX with the 2.4-liter four-cylinder engine.

While the 2017 Ford Fusion city number goes down, that vehicle received numerous updates to hybrid hardware and control software and over its 2016 counterpart that make the two figures not entirely comparable.

2017 Honda Accord Hybrid

2017 Honda Accord Hybrid

Several manufacturers have already started to point out the new cycle in their publicity materials.

A press release on the 2017 Honda Accord Hybrid, for example, includes this wording:

EPA fuel economy ratings of 49/47/48 (city/highway/combined) put the Accord Hybrid at the top of the class for ratings, making it the most powerful and fuel efficient midsize hybrid sedan in America.

These EPA fuel economy ratings are based on new, more stringent ratings requirements enacted by the U.S. EPA for the 2017 model year.

Based on the new requirements, Honda estimates the 2015 Accord Hybrid ratings would be 48/45/47 (actually rated at 50/45/47 under the previous method), indicating a +1/+2/+1 increase for the 2017 model over the previous version.

Whether the public will notice (or care about) such minor adjustments is debatable.

But the overall lesson, as EcoModder stresses, is that 2016 and 2017 numbers can't be directly compared without understanding the EPA's procedural changes.


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