The U.S. Environmental Protection Agency issued new guidelines to car makers this week, clarifying how they should measure gas mileage. Auto-makers like Hyundai, Kia and Ford have gotten in trouble for over-stating fuel-economy claims, and many consumers assume that the miles-per-gallon numbers on a new-car sticker must be taken with a heavy grain of salt.

However, the big surprise about those numbers is this: For most cars, they’re pretty accurate.

David Greene, a professor at the University of Tennessee, was one of the architects of the website fueleconomy.gov, where consumers report their actual mileage. Now, he’s looking at how close those thousands of reports are to the official mpg numbers.  

“They are pretty close— within a couple of percentage points— on average,” he says. “But that’s kind of like saying the average family has 2.6 children. Nobody has 2.6 children.”

As the saying goes, your mileage will vary, depending on how and where you drive. But the numbers reflect the average.

It wasn’t always this way, but in 2008 the E.P.A. changed testing procedures to reflect reality— including driving speeds of up to 80 miles per hour, air-conditioning systems running against 95-degree heat, and the like.

The result: the estimated mpg on most cars dropped by 10 to 20 percent.