ANN ARBOR, Mich. — The head of the U.S. Environmental Protection Agency said yesterday that he plans to propose new regulations for testing vehicle fuel economy by the end of this year and wants the new tests in place by the 2008 model year.

The update will be the first major change to the EPA's fuel economy tests since 1985. The EPA tests vehicles to get the estimated city and highway fuel economy that is displayed on vehicles' window stickers.

EPA Administrator Steve Johnson said he hasn't decided what the final proposal will look like, but it will consider more factors than the current test. People drive faster than they did in the 1980s and road congestion has increased. Vehicles have more accessories like air conditioning, which use up vehicle power, and the EPA still conducts tests at 75 degrees, which doesn't take into account the effect of hot or cold weather on fuel economy.

"Everyone agrees that the test procedures and our calculations are outdated. We've got 20 years of new technology, 20 years of experience, 20 years of new driving conditions," Johnson said.

Johnson made the comments during a tour of the EPA's National Vehicle and Fuel Emissions Laboratory in Ann Arbor.

There has been growing pressure to change the tests in Washington. This summer, the Senate agreed to require the EPA to update fuel economy tests to reflect real-life driving, saying fuel economy numbers are inflating performance by 10 percent to 30 percent. That amendment was later dropped by a House-Senate conference committee, according to the Union of Concerned Scientists, an environmentally oriented group that has been pushing for changes.

Automakers perform the EPA's tests themselves and post the results. The EPA visits automakers' labs and tests about 10 percent of all new models each year to make sure manufacturers are reporting accurate numbers.

Margo T. Oge, the EPA's director of transportation and air quality, said it's difficult to design a test that mimics real-world conditions because they vary so much. But she said the agency will use data collected from various states about real-world driving habits. She said the EPA also can require labs to raise or lower the temperatures to see how climate affects the tests.

She said the new tests could lower the posted fuel economy for some vehicles and raise it for others.

"The impact will vary from vehicle to vehicle," she said.

The Alliance of Automobile Manufacturers, which represents automakers, says drivers can influence fuel economy, as well.

For example, spokeswoman Eron Shosteck said, U.S. drivers could save 1 billion gallons of gas each year if they kept their tires properly inflated. Obeying speed limits also would help cut down on fuel use.