When it comes to fuel economy, the Environmental Protection Agency (EPA) ratings printed on window stickers are the holy writ for U.S. new-car buyers.

But those ratings come with the disclaimer "your mileage may vary" for a reason.

The difference between EPA-rated fuel economy and the results from real-world driving can be significant.

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Ford, Hyundai, and Kia recently had to lower ratings for certain models after customers found the original figures were unachievable outside the lab.

Yet it's also possible for fuel-economy discrepancies to work in a driver's favor.

Eight out of 10 drivers reported getting better fuel economy than the EPA ratings on their cars' window stickers in a new AAA study (via Automotive News).

The association conducted this latest study after finding that one third of U.S. drivers did not trust window-sticker mileage ratings.

That led AAA to analyze 37,000 records from the EPA's Fueleconomy.gov website, which lists the fuel economy of all new models, and allows owners to input their own estimated mileage.

In addition to surveying more than 8,400 year, make, and model combinations, researchers tested three vehicles--a 2014 full-size pickup truck, 2014 large sedan, and 2012 mid-size sedan--to confirm the EPA ratings were accurate.

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The vehicles were driven on Southern California roads and tested on a dynamometer.

In the end, the measured fuel economy matched EPA ratings, leading AAA to conclude that "driving behavior, vehicle condition, driving environment, and terrain" were the primary reasons for the discrepancies.

Certain types of vehicles also proved more likely to surpass their EPA-rated fuel economy.

2015 Nissan Altima

2015 Nissan Altima

Among the vehicles surveyed those with manual transmissions beat their EPA ratings by an average of 17 percent.

Drivers of diesel vehicles reported fuel economy that averaged 20 percent higher than EPA ratings.

That matches anecdotal evidence we've seen for some time showing diesels exceeding EPA figures--particularly in highway driving.

Sedans with naturally-aspirated V-6 engines got 9 percent better fuel economy, while gasoline V-8 pickup trucks beat their window-sticker ratings by 5 percent.

However, not all vehicles managed to exceed their rated fuel economy. Some did the opposite.

Drivers of Ford F-150 pickup trucks equipped with turbocharged EcoBoost V-6 engines reported gas mileage that was 9 percent below EPA ratings.

2016 Ford F-150

2016 Ford F-150

It was a similar story for sedans with turbocharged four-cylinder engines, which got fuel economy that was 4 percent worse than the window-sticker ratings.

Driving style can more significantly affect the performance of small turbocharged engines. They only deliver good fuel economy when they're driven economically.

The EPA announced in February that it will issue new guidelines for testing, which it hopes will cut down on discrepancies.

The guidelines more thoroughly address how cars should be prepared for testing, and the necessary conditions for certain tests.

So far they have not been made into legally-binding rules, a process that takes considerably longer.

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