A man reads the specifications of a Ford C-Max hybrid vehicle displayed on media preview day at the Los Angeles Auto Show on November 28, 2012 in Los Angeles, California. - 

Ford announced that its C-Max Hybrid car doesn’t in fact get 47 miles per gallon on average, but actually 43. The car company is being sued for misleading mileage and has offered to pay customers $550 to make up for the difference, at an estimated cost of 17 million dollars to Ford.

Here’s how it happened: Ford used the Ford Fusion hybrid to determine mileage for the C-Max, according to Dave Sullivan with Auto Pacific. "The C-Max is really a different vehicle than the fusion,” says Sullivan.

Obvious, but not illegal under EPA rules because the two cars share the same engine. And Ford isn’t the only one with mileage issues. “This is happening frequently now with hybrids,” says Michelle Krebbs, senior analyst with Edmunds. She says Honda and Hyundai have had similar problems, “and I don’t think this will be the last.”

Krebbs says despite some small reforms, the way the EPA and car companies evaluate mileage hasn’t changed a whole lot since the 70s. Now especially with more hybrids on the market, “the technology is outpacing the tests.” Hybrids do better in stop and go city driving because they recharge their batteries every time the car brakes, and mileage can then slip on the highway. That’s the exact opposite of traditional vehicles.

Driving habits and even temperature can have significant effects on mileage. Krebbs says when Edmunds took the C-Max out for a test drive they were able to get the 47 mpg that Ford just revised.

“It is very misleading,” says Auto Pacific’s Sullivan of the current system of mileage rating. “I’m hoping the EPA will find out a better way to do this.”

He says all mileage ratings, at this point, should be taken with a grain of salt.

The EPA has said it will take a hard look at that mileage loophole allowing one car to be a proxy for gas mileage in another.