The United States Environmental Protection Agency proposed new test methods for calculating the fuel economy estimates posted on the window stickers of new cars and trucks.

The city and highway miles per gallon estimates help people do one thing: figure out what kind of fuel efficiency their car or prospective car might get. It’s a factor in an increasing environmentally-minded market, and the numbers have been compiled since the 1970s.

The system hasn’t been updated in more than two decades, and recent tests by Consumer Reports found EPA window stickers could be off by up to 50 percent during normal driving.

EPA Administrator Stephen Johnson said the new testing method would come much closer to bridging “the gap between what the window sticker says and what the consumer can expect in their fuel economy.”

The EPA states current mileage estimates for all vehicles will be reduced by approximately 5 to 15 percent for highway driving and 10 to 20 percent for city driving – depending on the type of driving and kind of vehicle.

The new calculations will have the greatest impact on hybrid-electric vehicles, the agency said. These cars have become some of the automobile industry’s most sought after models now that gas prices have risen. But the nature of hybrid technology makes its fuel economy more sensitive to certain factors, like cold-weather driving and air-conditioning use. These cars come typically with relatively smaller power plants, but still have to heat and cool the same amount of space.

It’s not all downhill, though. Hybrids that run on a parallel system, like the Toyota Prius, are able to operate solely on their electric motors, so their city figures should remain relatively constant.

Automobile makers appear to be in support of the test changes.

“Circumstances have changed in the world since the rules were established,” Toyota spokeswoman Martha Voss said. “We want customers to get the most accurate information possible.”

There also is no reason to suspect lower mpg estimates will slow the auto consumer market, as the changes will affect most makes and models.

“There might be some changes in the comparisons,” NIU Economics Professor Stephen Karlson said. But “more meaningful information about gas mileage might make people more willing to buy cars.”

The new tests should begin being used for all 2008 model vehicles. The EPA will be hearing concerns about the changes for the next 60 days via their Web site, epa.gov.