(CBS 5) A Southern California driver has filed what could be the first class-action lawsuit against a hybrid carmaker over mileage claims.

John True of Ontario in San Bernardino County bought a 2006 Honda Civic Hybrid and sued the manufacturer after driving 6,000 miles.

True says that while the EPA mileage was advertised as 50 miles per gallon for combined city and highway, he only achieved 32 miles per gallon. True accuses Honda of misleading consumers.

Felix Kramer of Redwood City, who founded the nonprofit hybrid advocacy group calcars.org, says the plaintiff has a point.

"The car manufacturers stick to those advertised figures because it's the standard everyone agrees on," Kramer said. "But realistically, in the real world, no one's been getting those numbers because people drive faster and they drive with air conditioning, and both of those are not taken into account."

Kramer says he gets 35 miles per gallon in combined city and highway driving in his new Toyota Camry hybrid, a marked difference from his non-hybrid Camry.

"In the general, hybrids are getting somewhere between 50 and 75 percent better mileage per gallon than the equivalent that's not a hybrid," he said.

Kramer says the discrepancy in mileage estimates rests with the Environmental Protection Agency.

The EPA, which hasn't revised its testing for years, has overstated mileage numbers, and is only now adjusting them for the 2008 models.

"So they're bumping everything down about 20 percent," Kramer said.

So True's Honda Civic hybrid first advertised as 50 mpg combined is now listed as 42 and drivers say they're averaging 46, still higher than True's average of 32.

Honda says not all drivers will reach the maximum miles per gallon. It depends on driving conditions and habits.

But Kramer says it is possible to reach the old, higher EPA number.

"If they coast up to stoplights and don't do jackrabbit starts. if they use cruise control, they'll get better mpg," Kramer said. "If you bunch short trips into longer trips you'll do better because every time the engine starts and warms up the mpg goes down 50 percent of any posted number."

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