Why does EPA mpg differ from real numbers?

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The EPA estimates the city and highway mpg for all U.S.-sold vehicles. Image: TexasGOPVote.com

Because of EPA disclosure regulations, a new car buyer can leave a dealer’s lot confident that his or her new vehicle will clock in at the miles per gallon rated on its sticker, right? Maybe not always. But is that a cause for alarm, and what should be done about it, if anything?

‘Actual mileage may vary’

According to the Chicago Tribune, with gas teetering around $4 a gallon, if you expect 35 mpg out of your new car, but actual clock only 20 mpg, you could lose about $1,300 for every 15,000 miles you drive. However, just such discrepancies occur.

Eric Mallia, business manager for FleetCarma in Waterloo, Ontario, said:

“There are huge variances. We’ve done some analysis and seen up to 60 percent variance from window stickers. We find that’s something most people don’t understand.”

Three largest factors

David Greene and Zhenhong Lin, fuel-efficiency researchers at Tennessee’s Oak Ridge National Laboratory, identified some factors that cause significant variations between EPA mpg estimates and real-world numbers. Here are the top three:

No. 1: Amount of city driving versus highway driving. Stop-and-go city traffic can eat up fuel as much as 27 percent more quickly than on the freeway, according to the research.

Jim Trainor, a spokesman for Hyundai, said:

“If you’re (driving) around town and going a mile here, a mile there … you’re not going to get anything close to the EPA’s test ratings.”

And just to make it a little more confusing, the opposite may hold true for hybrids. They use an electrical system to stop and start the engine in town, which greatly improves fuel economy at slow speeds.

No. 2: Driving style. Aggressive driving can lower fuel efficiency by as much as 18 percent, says the Oak Ridge numbers. Edmunds estimates that discrepancy nearly twice as high, at 35 percent.

No. 3: Air conditioner use. The AC can make a 14 percent reduction in fuel economy, says the Oak Ridge research.

Fuel type matters

The type of fuel used also affects fuel efficiency. The EPA uses gasoline without additives in its testing, whereas most commercial fuel sold in the U.S. contains 8 to 10 percent ethanol.

[One stop, buy here, pay here, car lot.]

Honda successfully sued

The discrepancy between EPA estimates and real-world mileage numbers came to a head in February when Heather Peters was awarded nearly $10,000 in damages from Honda for the under-performance of her 2006 Honda Civic Hybrid.

That case started in 2007 when a man named John True sued Honda because his mileage did not match the EPA estimate. That effort became a class-action lawsuit, in which each participant was awarded either $100 or a $1,000 discount off a new Honda.

Peters, a former corporate lawyer and the owner of a 2006 Civic Hybrid, opted out of the class-action suit. Instead, she took her case to small claims court where she could sue for up to $10,000. And also where, by law, Honda is unable to bring its high-priced attorneys.

Estimates are estimates

So, in conclusion, the EPA sticker-estimated mileage is not intentionally misleading, or even inaccurate. But estimates are estimates, and a host of factors affect real highway numbers. It is unlikely that those number will ever exactly match the estimates gleaned in controlled lab conditions.


Chicago TribuneĀ 
Green Auto BlogĀ 

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