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September 2007
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New mpg figures to reflect real-world conditions
The government’s new test is more accurate, but numbers are still off

Illustration of cars
Illustration by Mike Jones
The Environmental Protection Agency (EPA) is updating its methods for estimating the fuel economy of new vehicles. Starting with 2008 models, the window stickers that all new cars carry showing the estimated mpg will reflect the new ratings. The good news is that the numbers on the sticker should be closer to what you should expect to get in normal driving. The bad news is the predicted mpg will be lower than what people are used to seeing on the stickers.

Consumer Reports Video
FUEL ECONOMY
New MPG figures
The EPA’s change in estimating fuel economy is long overdue. Consumer Reports has been advocating for it for years. The old system, developed in the 1970s, does not accurately reflect today’s driving conditions, motoring habits, or vehicles. Many cars today spend 62 percent of their annual miles in stop-and-go traffic, where fuel economy is the lowest. Faster speeds are another big factor. The national highway speed limit was 55 mph in the mid-1970s, but vehicles now burn up to 15 percent more fuel per mile simply by traveling faster. Air conditioning, much more prevalent on new cars than it was 30 years ago, is also a factor. Running an air conditioner at 65 mph can reduce your fuel mileage by about 1 mpg and possibly more if you drive faster. The old EPA tests were conducted without the air conditioning turned on.

The new system is designed to address all of these changes. Additional tests will be run at higher speeds and while using the A/C, and will include a closer look at stop-and-go driving and more aggressive starts and stops. Cold-temperature tests are also being added to more realistically show what drivers in northern states can expect in winter.

Consumer Reports thinks the change will benefit consumers even if the lower numbers might be painful to look at in the showroom at first. A 2006 CR study showed that cars seldom meet the agency’s claims with the old system. We compared EPA figures with the mileage we measured in 303 cars and trucks for model years 2000 through 2006, and included mainstream, high-volume vehicles in a wide range of makes and models.

Our results showed that about 90 percent of the vehicles returned worse mileage than the EPA estimates. Gasoline cars averaged 9 percent fewer mpg. The biggest differences we found were in city driving. On average, the mileage was about 30 percent less than the EPA numbers. Our highway mpg more closely reflected the EPA ratings.

Consumer Reports tests fuel mileage in controlled conditions and in the real world. We use precise fuel meters spliced into the cars’ fuel lines, and adjust for outside temperature in our calculations. City mileage is conducted on our track using a 1.1-mile loop of stop-and-go driving combined with idling to simulate traffic and stoplights. Highway tests are done on a section of local interstate, run in both directions to minimize the effect of wind. We also drive all the cars on a 31-mile route that includes a mixture of highway, rural roads, and around-town driving. All tests are run several times using multiple drivers, and our overall mileage figure is an average of these tests.

While the new overall mileage ratings are closer to the results of our real-world testing, there are still discrepancies between the EPA’s lab-generated results and the mileage we got from driving the cars on roads. With the new system, our overall mileage is actually better than what the EPA predicts for some models.

Until the new system is in place, you can compare the old and new ratings online. The EPA is posting both mileage estimates for models going back to 1985 as well as an explanation of how their methods have changed, at www.fueleconomy.gov. Click on “New fuel economy ratings” on the home page. Fuel mileage for all models that Consumer Reports has tested can also be found in our individual vehicle profiles.