It's a different kind of sticker shock, but a new method of assessing fuel efficiency is supposed to add a bit more reality to the purchase of a car or truck.

It turns out the miles-per-gallon estimates on those big stickers pasted on the windows of new vehicles have been largely theoretical.

"They need to be honest about that kind of stuff," said Larry Ragan of Ojai as he scanned the sticker on a black Prius parked at Toyota of Ventura on a clear brisk day last week.

Ragan knew when he bought his big Toyota Sequoia four years ago the mileage wasn't great, but it wasn't from looking at the advertised mileage.

"I looked that kind of stuff up, because it's never what they say it's going to be," said Ragan, who is considering buying a new car in the near future.

For years, many drivers have known that few cars lived up to even modest promises of city and highway miles per gallon. Now, customers are seeing a bit more truth in advertising.

The U.S. Environmental Protection Agency last year ordered new testing procedures to produce more realistic fuel-economy estimates beginning with 2008 models, which are now on car lots.

Steve Mazor, chief automotive engineer for AAA, said the change is good for consumers.

"No matter what they did, a driver could almost never get that kind of mileage," he said. Mazor and his team of scientists at the automobile association found the advertised numbers on most vehicles were as much as 30 percent off actual performance.

Consumer Union, the nonprofit group that publishes Consumer Reports, reached similar findings that showed about 90 percent of vehicles had worse gas mileage than advertised. For instance, the Toyota Prius, listed as the most fuel-efficient car this year, had an advertised miles per gallon of 48 in the city, but it got closer to 35 miles per gallon.

Ed Whetstone of Thousand Oaks traded in his 2007 Chevy Tahoe last week for a new Prius. "We liked (the Tahoe) but our driving changed," said Whetstone, who runs a janitorial company.

He's found the advertised mileage on the new Prius is pretty accurate. "I haven't gone through a tank of gas yet, but it's pretty close," he said.

In the past, he had "always figured a car's mileage was 10 to 13 percent less than what they said it was."

There was an exception with the couple's BMW 550. "It gets exactly what it said on the sticker."

Paul Choi, who handles Internet car sales for Toyota of Oxnard, said he had customers bringing in their cars wondering why they weren't getting better mileage.

"These are more realistic figures, and it's a way of protecting consumers," Choi said.

The problem with the old mileage tests was just that — they were old, based on an almost 40-year-old method, said Frank Markus, technology director for Motor Trend magazine. Markus, who has written in detail about the changes, said it's not just the tests that are different now. People drive differently from the 1960s and '70s — we are the lead-foot generation.

The old tests didn't measure the kind of full-throttle, air-conditioned, stop-and-start driving of today, Markus said.

"Actually, we've found the new numbers are much more realistic," said Markus. "I have to take my hat off to the government."

The EPA has not yet tested every vehicle. The costs and time it would take would have derailed the program. But piggybacking on real-world driving tests for emissions and using other data, government scientists were able to deduct realistic mileage estimates, he said.

Most of the estimates have translated into 11 percent to 15 percent drops in rated efficiency, Markus said. For hybrids, city mileage estimates have dropped 18 percent to 22 percent, he said.

Whether any of this will have an effect on consumer choices remains to be seen.

Jon Osborne, research director at the Westlake-based marketing research firm J.D. Power and Associates, said gas mileage is at the top of the list of mentioned reasons for buying a car. But the company's recent "Avoider Study" found perceptions were often more important than actual fuel efficiency.

Osborne cited General Motors' Hummer as an example. The perception is the big SUV is the poster child for gas-guzzling vehicles, but it actually compares well with other trucks and sport utility vehicles, Osborne said.

If the new mileage estimates are more accurate and consumers rely on them in making decisions about future purchases, it might help cars that get a bad rap. But for those on the other end of the spectrum, like the Prius, where fuel-efficiency estimates have dropped, it's unclear how buying habits might be affected.

As he walked through a car lot at the Ventura Auto Center, Ragan said he would be looking closely at mileage estimates when he and his wife buy their next car.

The couple are leaning toward a bigger car for their family snowboarding trips. Ragan said he's interested in a hybrid GM Tahoe, a sport utility vehicle.

GM, the beleaguered American giant, hopes to reclaim its dominance in North America with a new fleet of hybrids, plug-in hybrids, fuel-cell cars and extended-range electric vehicles, GM officials said.

With the cost of gas and the effect on the environment, Ragan said car manufacturers and drivers just can't avoid focusing on fuel efficiency. "I think you have to now," he said.