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How Are Fuel Economy Ratings Calculated?

A Quick Look at EPA Fuel Economy Rating Calculations

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Manufacturers and the Environmental Protection Agency (EPA) test a vehicle's fuel economy ratings in far less than real-world conditions. The vehicle is placed on a dynamometer, a set of rollers that lets the car or truck sit in place while it turns the vehicle's wheels.

The drag on the rollers is adjusted to simulate different driving conditions, but the vehicle are never tested in the real world -- without wind resistance and without any accessories running. To top it off, up until 2008 simulated speeds were a lot less than the average driver drives -- an average of 20 miles per hour for city tests and 48 miles per hour for highway conditions.

EPA Fuel Economy Ratings Calculations From 2008 Onwards

For 2008 and onward, the EPA has adjusted its testing requirements to simulate higher driving speeds, faster acceleration, running a vehicle under colder temperatures -- and the opposite, operating a vehicle with the air conditioning on.

The additions have reduced mileage projections. Results are expected to be somewhat closer to the actual mpg estimates listed on the sticker, but don't expect them to be right on target. Estimates will still be higher than most of us will see in real life, because it's impossible to duplicate our actual driving habits in a laboratory.

Why Can't I Find EPA Ratings for Some Trucks?

The EPA doesn't require manufacturers to test vehicles with a gross vehicle weight rating (GVWR) over 8,500 pounds -- the category some large trucks and SUVs fall into.

EPA Ratings - the Bottom Line

EPA ratings can help you compare vehicles in the same class, but don't count on them for an accurate projection of your actual gas mileage.

EPA Fuel Economy Ratings Prior to the 2008 Model Year

Use the EPA's calculator to convert old mpg estimates to the new estimates.

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