WASHINGTON, D.C. --- Federal regulators this week proposed a new way to measure fuel economy in cars and trucks, in hopes of bringing estimates more in line with real-world everyday driving experience. The proposed testing system could cut fuel-efficiency estimates for many vehicles by 20 percent in cities and 15 percent on highways, according to a report in the Los Angeles Times. Gasoline-electric hybrids are expected to see the biggest shifts in mpg estimates. If approved, the system would represent the first change to the Environmental Protection Agency's mileage test methods in 20 years. The new proposal calls for additional lab tests that would simulate high-speed driving, rapid acceleration, and stop-and-go driving in gridlock traffic, the Times reported. Testing would also, for the first time, take into account the effects of starting vehicles during cold weather and running air conditioning during hot weather. Testing could also measure the fuel-efficiency impact of accessories such as power windows and even DVD players. Current EPA tests rely on lab tests that simulate driving in mild weather at top highway speeds of 60 mph and average speeds of 48 mph. In the lab environment, test administrators mount vehicles on dynamometers, which spin wheels to calculate mileage. The EPA estimates are listed on price stickers of new vehicles. In last year's energy bill Congress mandated testing changes, spurred by complaints from consumers and environmentalists that mpg estimates are routinely off the mark. The Automobile Club of Southern California's test center in Diamond Bar, Calif., played a key role in developing the newly proposed testing methods, the Times reported.