The American Automobile Association plans to use real-world fuel economy tests to prove the U.S. Environmental Protection Agency is overestimating the miles-per-gallon rates on dozens of new cars. The AAA is endorsing the "Fuel Efficiency Truth-In-Advertising Act of 2005," a congressional bill introduced this week that would require the EPA to update its gas-mileage tests, the association said last week. The EPA's current mileage ratings are based on 30-year-old tests that fail to account for such modern factors as air conditioning, increased traffic congestion and the fact that many trips never go far enough to warm up the engine, say AAA officials. The vehicles never exceed 60 mph and never accelerate hard enough to hit 60 mph from a standstill in less than 18 seconds and always go far enough to fully warm up the engine. EPA uses correction factors to try to get closer to actual mileage. After testing hundreds of new vehicles, AAA found dozens of vehicles with overestimated mileage, in some cases by as much as 10 mpg. AAA's mileage results came from tests that included such driving conditions as stop-and-go traffic, climbing steep grades and operating vehicles loaded with groceries, according to the AAA. The agency is studying to what extent such factors will lower fuel economy and it plans to establish testing changes within the year.