ST. LOUIS – More than 31 million travelers are expected to hit the nation’s highways this Memorial Day weekend, none of them wanting to spend more money than they must at the gas station.

Many of those drivers won’t see the fuel efficiency promised when they bought their vehicles, say several consumer and environmental groups.

The government’s method to calculate fuel economy is 30 years old, and does not consider today’s busier highways, higher speed limits and increased use of air conditioning.

So the Environmental Protection Agency is re-evaluating the way it calculates fuel efficiency to make sticker information on new vehicles more reliable.

“We believe consumers should have the most accurate information possible when it comes to expected gas mileage of the vehicles they purchase,” Susan Pikrallidas, vice president of public affairs for AAA, said in a statement. “This would be accomplished by requiring EPA to use real-world tests in setting federal mileage estimates.”

Every time gas prices go up by a nickel or so per gallon, St. Louis area service stations get calls from drivers worried about fuel economy.

A lot of motorists don’t know how many miles per gallon they are supposed to get. But enough pay close attention to the fuel sticker and want a mechanic to tell them why they aren’t getting the 25 miles per gallon they had expected.

“They just ask if there was a way we could check their fuel mileage per gallon,” said Annette Stein, who answers phones at Brentwood Volvo.

The Senate passed a highway funding bill in May that contains a provision requiring the EPA to make its fuel economy tests more realistic. The process already was under way, said agency spokesman John Millett, and it will continue regardless of whether President George W. Bush signs the bill.

“We have a lot of data to look at and from analyzing that data, we hope we can make appropriate changes to make the window stickers more accurate,” Millett said.

The automobile association says its own tests show the stickers could be off by as much as 30 percent on some models.

Manufacturers currently are required by the agency to test new models for fuel efficiency on treadmill-like machines called dynamometers inside their own test labs.

Professional drivers take the models through 30-year-old driving cycles resembling city and highway conditions. The cycles are standardized and designed by the EPA. The agency confirms about 10 percent to 15 percent of the manufacturers’ calculations inside its own lab in Ann Arbor, Mich.

Because the tests are conducted in a controlled environment, they don’t account for many factors drivers encounter daily, such as weather, driving with the windows down and using the gas-consuming air conditioner.

The highway test is a 10-mile drive at an average speed of 48 mph. Unlike rush-hour in a large city, there is little idling and no stops. The test simulating city driving is about 11 miles with 23 stops, with 20 mph as the average speed.

A lot has changed since the driving cycles were designed in the 1970s: congestion is worse, highway speed limits are higher and most automobiles now have air conditioning.

“That’s part of what we’re looking at now,” Millett said.

In March, AAA published a list of 39 vehicles that its testers put through a range of driving conditions, including stop-and-go traffic, steep grades, trips to the supermarket and a mix of highway and city driving.

The organization’s tests were neither standardized nor scientific, AAA said, but reflect more realistic fuel use. The 2003 Toyota Tundra, for example, got 12.9 miles per gallon in the AAA test, verses the 14 to 17 miles per gallon listed on the fuel sticker.

Some say the arguments from consumer and environmental groups are exaggerated and that the current fuel stickers give motorists an accurate enough number to base a purchasing decision.

“I don’t see how they can make them more accurate,” said Ron Reiling, executive director of the Alliance of Automotive Service Providers of Missouri.

“If you drive in rush-hour traffic everyday from O’Fallon, Mo., to St. Louis, your fuel economy will probably be less than what the sticker says it is. But if you’re driving from St. Louis to Kansas City, it will probably be right.”

The difference between expected and realistic fuel economy can prove costly at the pump, said Russell Long, executive director of Bluewater Network, the San Francisco-based environmental group that first petitioned the EPA for changes in 2002.

“It could be on the order of $400 to $500 per vehicle per year above what the driver anticipated,” he said.

The EPA expects to propose changes this year. The Association of International Automobile Manufacturers, based in Washington, supports the federal effort to reevaluate testing procedures, but is concerned that new rules might expand testing burdens on the manufacturers.

No test can be typical of most driving conditions, said John Cabaniss, the director of environment and energy for the manufacturers’ group. Traffic conditions change constantly and all highways and city streets are different.

“You have to have a standard cycle to do these things, but any cycle you pick is not going to be typical of every city,” he said. “The best you’ll ever be able to do is compromise.”

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