What should your real gas mileage be?

Fiat 500
Jerry Edgerton

Now this is getting really serious about gas mileage: A California woman is suing Honda in small claims court because she says her 2006 Honda Civic Hybrid never came close to the 50 MPG promised by the company. You may not be that commited, but if you are a new car shopper trying to estimate future fuel costs you will wonder: How close am I likely to come to the MPG figures on the window sticker?

Most new vehicles are capable of approximating those numbers under the right conditions. But that is a big caveat. "So much depends on how you drive and where you drive," cautions Jack Nerad, executive editor of the Kelley Blue Book. "If you are an aggressive driver you will not get the posted mileage. And your mileage will certainly vary if you drive mostly in the city or mostly on the highway."

The federal numbers posted at fueleconomy.gov, and overseen jointly by the Environmental Protection Agency and the National Highway Traffic Safety Administration, have moved closer to reality since the testing and rating program began in the 1970s. Now the tests include hot and cold weather conditions with air conditioning or the defroster running as well as somewhat sharp acceleration and braking. That was not the case prior to the 2008 model year. Dan Edmunds, director of vehicle testing for Edmunds.com, points out that the 2006 Honda Civic Hybrid owned by litigant Heather Peters dropped from a combined city/highway rating of 50 MPG to 42 with the testing changes for the 2008 model year even though the power plant remained unchanged.

Edmunds has a long-term test fleet of about 20 cars it keeps for at least a year. Most of those models come within one or two MPG of the combined rating over a year while being driven by 20 or so different drivers, according to Dan Edmunds. He notes that the vehicles most likely to fall short are those that are relatively underpowered. He cited a GMC Terrain large SUV with a four-cylinder engine instead of the usual V-6 or V-8 that missed the combined rating by 6 MPG.

Aggressive driving habits especially penalize the mileage results of hybrids, Edmunds adds. Accelerating swiftly minimizes the time the high-mileage electric motor is propelling the car. And sharp braking leaves less time for so-called regenerative braking, which recharges the battery.

The Web site fueleconomy.gov, in addition to giving city, highway and combined ratings, cites also what owners report to be their measured mileage. The web site Green Car Reports, while cautioning that owner-reported mileage is not verified, has compiled five cars that seem to exceed their EPA ratings. That list:

-- 2012 Volkswagen Jetta four-cylinder with manual transmission. EPA 34 MPG, owners up to 52 MPG

-- 2012 Honda Insight Honda's dedicated hybrid is rated at 42 MPG, owners report up to 52

-- 2012 Smart ForTwo: EPA 36 MPG, owner up to 46

-- Fiat 500 with manual transmission: EPA 33, owners up to 47

-- 2011 BMW 335d: This diesel model has EPA ratings of 27 MPG, owners report up to 38

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If you are shopping for a new car or may soon, here are some helpful ways to think about mileage of the models you consider:

-- Match the vehicle to driving conditions. Jack Nerad of Kelley Blue Book points out that hybrids make much more sense for owners who do mostly city or suburban driving. Stop-and-go conditions are ideal both for operating frequently on the electric motor only and for regenerative braking. In fact, gas-electric hybrids typically have higher MPG ratings for city driving than for highway cruising.

-- Take into account how you drive. If you like jackrabbit starts, quick stops and frequent lane changes, knock at least 10% to 15% off the MPG ratings as the likely penalty for your driving style.

-- Consider changing your driving style. If you care enough about mileage and saving money, slow down. It not only will boost your mileage, it probably will reduce your stress level.

Above all, don't expect to match the rated mileage every day you drive. Even if you hit it sometimes by driving carefully, traffic and weather conditions and other matters beyond your control can scale back your MPG sharply.

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    Jerry Edgerton, author of Car Shopping Made Easy, has been covering the car beat since Detroit companies dominated the U.S. market. The former car columnist for Money magazine and Washington correspondent for Business Week, Edgerton specializes in finding the best deals on wheels and offering advice on making your car last.