Do all automakers cheat on their MPG tests?

It's getting harder to trust those gas mileage numbers you see in government ratings and on the window stickers of new cars.

Revelations yesterday that Mitsubishi cheated on gas mileage testing, following Volkswagen's (VLKAY) diesel emissions testing scandal, raises questions about the honesty of the MPG ratings and emissions data reported to car shoppers.

"While I wouldn't describe fuel economy manipulation as rampant throughout the industry, it's obviously a recurring issue, one that will have regulators reevaluating their testing and certification procedures," said senior analyst Karl Brauer of Kelley Blue Book.

The Mitsubishi cheating issue is different from Volkswagen's emissions deception. Mitsubishi employees reportedly changed calculations on wind and tire resistance in testing to boost the MPG of its microcars sold only in Japan. But top company executives said the testing method was used on other models as well. Mitsubishi is further investigating whether exported models -- including those to the U. S. -- are affected.

In the Volkswagen case, a "defeat device" was installed on diesel models so that the car's sensors could detect when it was being tested on a machine rather than driving on the road. During testing, the device would turn off emission controls for smog-causing nitrogen oxides. That boosted the MPG. On Thursday, VW agreed in principal to a deal with the U.S. government on how it would compensate owners of the affected diesel models.

The issue didn't start with Volkswagen. In 2012, U.S. regulators fined Korean corporate siblings Hyundai and Kia a combined $300 million for manipulating fuel economy numbers. And Ford (F) paid up to $1,050 each to about 200,000 owners of six different models -- most of them hybrids -- whose MPG ratings it overstated.

"The testing procedures have not kept up with the manufacturers' creativity," said Rebecca Lindland, a senior analyst for Kelley Blue Book.

Part of the problem -- in the U.S. as well as in Europe and Japan -- is that the automakers do their own testing for gas mileage under standards set by regulators. This is unlikely to change. "It would be nearly impossible for the EPA to do all the tests themselves without an immense amount of funding," said Jessica Caldwell, director of industry analysis for Edmunds.com. Given the current political climate in Washington, more funding for the Environmental Protection Administration is highly unlikely.

More likely is further updating of EPA regulations and enforcement as these issues arise. For instance, after the Ford hybrid problem, the agency changed the rule that test results of one model could be applied to another model using the same engine. That issue had caused the Ford overstatement.

So, if you're shopping for a new car, how should you view the gas mileage ratings you see on company websites or window stickers?

  • Believe the disclaimer: Your mileage may vary. Website Green Car Report said its surveys show that for most cars, the EPA combined city-highway figure is within 10 percent to 15 percent of real-world mileage.
  • Be especially skeptical of extremely high mileage claims compared with competitors. Ford courted trouble by advertising its 2013 C-Max hybrid as getting 47 MPG in both city and highway driving when buyers were reporting gas mileage in the 30s. The EPA required Ford to restate its MPG numbers.
  • Don't just look at manufacturer's ads, which typically promote the highest number for highway driving. Go to the EPA site, fueleconomy.gov and look at the city, highway and combined ratings. The site will show you different ratings for different engines that are available for the same model -- something a salesperson may not make clear.

As for the automakers, those overstated ratings probably aren't worth it in the long run. "Fuel economy is an important metric for car shoppers," said Edmonds' Caldwell, "and automakers who cheat the system are only creating ill will among buyers." Just ask VW.

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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.