DETROIT (CNNMoney.com) – Soon all vehicles in the United States will get lower fuel mileage as estimated by the Environmental Protection Agency if the agency has its way.

It's not that vehicles will actually be less fuel efficient. It's just that the EPA wants to bring its estimates closer in line with reality.

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"With President Bush encouraging energy conservation, EPA is ensuring American motorists can be confident that the fuel economy estimates more closely reflect today's real world driving experiences," said EPA Administrator Stephen L. Johnson.

Under a proposal announced Tuesday, the EPA's fuel mileage testing regimen will include tests that mimic the effects of high speed and rapid acceleration, use of air conditioning, and cold temperatures.

The EPA will also include an across-the-board adjustment of fuel mileage estimates to account for a variety of factors not included in the tests, the agency said.

Under the new methods, the city fuel mileage estimates for most vehicles would drop 10 to 20 percent from today's estimates, depending on the vehicle. The highway mileage estimates would generally drop 5 percent to 15 percent, the agency said.

The last time changes were made to EPA fuel estimate methods was 1985 when an adjustment factor was introduced to bring estimates closer to real-world figures. The agency has opened a 60-day comment period on the proposal.

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