The Environmental Protection Agency will replace its testing procedures for fuel-economy estimates with a system that better reflects how motorists drive today.

The planned changes follow complaints from consumers who have been paying more attention to the accuracy of the estimates as gas prices have spiked.

The EPA said the tests will take into account faster driving, more idling in traffic, and more abrupt acceleration and braking. Changes to the rules have been considered long overdue by consumer groups and some automakers. The procedures were established in 1977 and don't take into account changes in automotive technology such as the prevalence of air conditioning.

"A lot has changed -- things such as speed limits, a lot more cars with air conditioning and other equipment," EPA Administrator Stephen L. Johnson said Friday. "Clearly, people do a little bit more stop-and-go and probably accelerate more than assumptions" behind the original standards, he said.

Johnson said a new system could go into effect in two years.

Johnson spoke during a tour of the EPA's National Fuel and Vehicle Emissions Laboratory in Ann Arbor. He said he was there to learn more about testing as the agency moves to complete new procedures by year-end.

By law, EPA fuel-economy information is provided in window stickers on new cars and trucks. Legislation approved this year by Congress requires the EPA to test under more real-world conditions.

The original rules were put in place after the oil price and supply shocks of the 1970s. High oil prices this year have again raised awareness of gas mileage. Some consumers have been particularly upset with the government's ratings on gas-electric hybrids, such as the Toyota Prius and the Ford Escape. Some owners of such vehicles have said they get far lower city mileage than the EPA estimates because of quirks in the testing system.

David Champion, director of automobile testing at Consumer Reports, said the EPA's numbers can differ from actual mileage by 50 percent and sometimes more. He also said automakers have learned how to optimize their vehicles to do well in EPA tests.

The EPA does not test vehicles for fuel economy. Instead, it issues guidelines that automakers use to test their own vehicles. Agency officials said the EPA audits 10 percent of the tested vehicles for accuracy.

The EPA laboratory Friday looked like a cross between a used-car lot and a science fair as its engineers showed off their best work to their boss. Technicians in long blue coats wheeled around carts of equipment. The engineers had lots of graphics and charts for Johnson, some perched on the exposed body of a giant delivery truck that had been dismantled.

Although it was his first trip to the lab, Johnson seemed at ease. He is a 25-year veteran of the agency and the first scientist to lead the EPA.

The Ann Arbor lab was established in 1971. It employs about 400 people, mostly scientists and engineers. Besides the fuel economy estimates, the labs establish national pollution emissions standards for cars and trucks.

Johnson said the EPA intends to propose the new testing rules by the end of the year.

The proposal would be subject to a 90-day public comment period before it took effect.