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SAN FRANCISCO (KGO) — A lively debate over the real gasoline mileage offered by one particular car is now jumping from the Internet to the courthouse. "Your mileage may vary" has become a catch phrase and often a punchline to a joke. But it is no laughing matter, as a legal fight begins over those EPA numbers and how they are advertised.

Louis Bird drives a 2011 Hyundai Elantra. He keeps close track of it; every time Bird buys gas, he writes down how much he bought and how far he's gone.

"At about 26,000 miles I have an overall mileage rating of 29.92," he said.

And that he says is the problem. The sticker that came with his car promised 29 city miles per gallon and 40 on the highway. The smaller print showed the city mileage could go as high as 34 and highway as much as 47 mpg.

"My mileage isn't anywhere close," Bird said. "I've tracked every single gallon of gas I that put into this car from when I got it."

There is a lively debate about just how many miles per gallon the 2011 and 2012 Elantras get. Consumer Reports finds the mileage to be very good, saying the cars got nearly 40 miles per gallon in their tests. Popular Mechanics published a report saying its test of the Elantra, "demonstrated significantly better fuel economy… approaching 50" miles per gallon highway and in the mid-30s for city driving.

But Birdand attorneys with Consumer Watchdog are unmoved. They have filed a lawsuit claiming the advertisements for the car concentrate on the "40 miles per gallon" and don't follow federal rules about justifying and clarifying that number.

"The point isn't whether 1 out of 100 people could get 39 miles per gallon, the point is that 99 out of 100 people are not likely to get it and yet the car company sold the cars based on the fact that every customer would get it," Consumer Watchdog spokesperson Jamie Court said. "That's the point."

7 On Your Side contacted Hyundai Motor America and it issued a statement saying it "…stands behind the fuel economy ratings obtained for the Elantra — ratings that have been repeatedly substantiated by reputable independent testing (AND) by the driving of legions of satisfied buyers, and by the EPA itself. HMA also stands behind the accuracy of its advertisements for the Elantra. HMA intends to vigorously contest Consumer Watchdog's allegations."