EPA shows new lower mileage ratings

NEW YORK (CNNMoney.com) -- Whether you're looking at a gas miser or gas guzzler, fuel economy is about to look a lot worse, according to new estimates from the Environmental Protection Agency.

The federal agency that creates official fuel mileage estimates for every new car and light truck sold in the United States is changing the way it creates those numbers. The new method will start with the 2008 models hitting showrooms later this year. New tests are supposed to more accurately reflect modern driving conditions and habits.

The EPA is cutting its mileage estimates for most vehicles.

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The new tests use faster speeds and quicker acceleration as well as air conditioner use and colder outside temperatures, all of which can cut into mileage. The estimates will also be adjusted downward to account for factors that are difficult to replicate in a laboratory, such as wind resistance and road surface friction.

The EPA has now released estimated figures showing how 2007 models would have been rated using both the both old and new methods. The new numbers are based on estimates of the difference made by new test procedures.

Most vehicles will end up with lower mileage estimates, whether they are fuel-efficient gas-electric hybrids or big-engined trucks and performance cars.

The Toyota Prius, the nation's best-selling hybrid model and the most fuel-efficient, is estimated to see its mileage rating drop 20 percent to 48 mpg in the city from the old estimate of 60 mpg. Its estimated highway mileage should fall almost 12 percent to 45 mpg from the current 51 mpg. The combined mileage estimate will fall about 16 percent to 46 mpg.

The Prius will remain the best-mileage car in the country even with the new formula. The big drop is not a surprise; Toyota (Charts) has been saying publicly for years that it believed the EPA estimates were too high for hybrids.

Toyota officials said Friday they're not concerned about the drop in the Prius's estimated mileage because almost all other vehicles' are dropping as well.

In fact, the Prius will appear to save more fuel, compared to the average vehicle, than it does now.

Using current mileage figures, the Prius uses 326 gallons less fuel over 15,000 miles than the average car, which is now estimated to get 25 mpg. Using the new figures, the Prius uses 346 gallons less than the average car, which is estimated to get 22 miles per gallon.

"The [EPA mileage] label shows the estimated fuel cost," he said.. "Buyers who are comparison shopping can clearly see that the Prius is now a better deal because it'll save more fuel."

It's not just hybrids that will see significantly worse fuel economy under the new formula. The Toyota Yaris, a subcompact gasoline-only car, will get its city mileage rating cut nearly 15 percent to 29 mpg from 34. Its highway estimate will drop 10 percent to 36 mpg from 40 and its combined estimate will fall about 13.5 percent to 32 from 37 mpg, according to the EPA.

The percentage declines won't be quite as bad on the other end of the fuel economy spectrum, but there will be double-digit drops in mileage there as well.

The Lamborghini Murcielago has the worst-rated mileage among 2007 models. Under new testing procedures, fuel economy for the manual transmission version are estimated to drop 11 percent to 8 mpg in the city from 9 previously, with highway mileage falling 7 percent to 13 mpg from 14 mpg in the earlier estimate. The combined rating for 2008 is expected to be 10 mpg, down 9 percent from 11 mpg previously.

Of course those spending about $300,000 on a car probably aren't worried too much about mileage.

Among gas guzzlers with wider ownership, the four-wheel drive version of the F150 pickup, the nation's best-selling vehicle from , is estimated to have its city mileage rating drop 14 percent to 12 mpg, while its highway mileage estimate will drop about 11 percent to 16 mpg. The combined rating should stand at about 14 mpg, down about 7 percent.

The Dodge 1500 Ram pickup truck, the rival to the F150 from DaimlerChrysler (Charts), will see its overall mileage trimmed 6 percent to 15 mpg, and its city mileage fall 7 percent to 13 mpg. Its highway mileage is expected to stay unchanged at 18 mpg.

Meanwhile, General Motor's (Charts) Chevrolet Tahoe large SUV is expected to see its combined mileage estimate cut by one mile per gallon to 16.

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