Who taught YOU to drive?

On fuel mileage, what you’re sold is not what you’ll get

A bit of bad news for drivers: Your new car doesn’t automatically get the lofty miles per gallon you see advertised. In fact, you may never hit that number. A bit of bad news for drivers: Your new car doesn’t automatically get the lofty miles per gallon you see advertised. In fact, you may never hit that number.
By Peter DeMarco
July 31, 2011

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After driving a big, bulky SUV for the past decade, I was determined to go green when buying a new car this spring. But three months into owning my “fuel-efficient cross-over SUV,’’ it feels, well, olive at best.

Ads for my new Chevy Equinox claimed it could get 29 miles per gallon on the highway. But on two lengthy trips to New York City, I’ve averaged 24.5 miles per gallon and 23.7 miles per gallon, respectively.

Those numbers are way better than the 12 miles per gallon I was getting with my old Chevy Tahoe. But I can’t help but feel disappointed . . . and curious about how car manufacturers and the Environmental Protection Agency come up with estimated miles-per-gallon figures.

At what speeds are cars tested on the highway? Is my own driving to blame for my subpar mileage? How much stock should consumers put in such estimates, anyhow?

‘Estimated’ Miles per gallon has been a trendy topic of late. The Obama administration would like us all to be driving cars that get around 40 miles per gallon by 2025. (The “54.5 m.p.g.’’ goal you may have read about is an artificially inflated number; with proper adjustments, it’s closer to 40 miles per gallon.)

By next year, new cars will come with EPA fuel economy and environment stickers that show in dollars and cents how much you’ll save by buying a greener vehicle.

With the new labels, car buyers will probably pay even more attention to miles-per-gallon estimates, which means accuracy will be more important than ever. And I have no doubt that the EPA’s testing procedures, though somewhat limited, produce accurate results.

But after more than an hour on the phone with EPA officials, and another two hours spent with fuel-economy experts from car advice website, I have some bad news for most of you: Your new car doesn’t automatically get the lofty miles per gallon you see advertised.

In fact, you may never hit that number.

“The highway number is the best you can get under ideal conditions, so my advice is forget about that number,’’ said Phil Reed, senior consumer advice editor at “The combined city/highway number is fairly close to what you can expect to get if you’re an average driver. We have found that the combined number is, say, plus or minus 5 percent accurate.’’

The EPA doesn’t contest this. In fact, the agency posts numerous disclaimers both on its website,, and car window stickers stating how the miles per gallon you actually get will vary from car to car and from driver to driver.

The disclaimers don’t immediately explain, though, just how many variables affect your miles per gallon.

Are your tires inflated? Do you have a clean air filter? Is your engine “broken in,’’ which happens around the 4,000-mile mark?

If you drive aggressively, breaking fast and flooring it to accelerate, your fuel economy will suffer. If you drive above 70 on the highway, your miles per gallon will suffer. Just using your air conditioner can reduce numbers by half a mile per gallon, the EPA says.

Then there are the variables you simply can’t control: Your car won’t perform as well on mountains; and gasoline has more oomph in the summer than in the winter.

“The fact that some people are buying cars in Alaska and some people are buying them in Arizona, with one car running at 10 degrees versus 110 degrees, simple physics and engineering are going to dictate that those vehicles are going to operate differently,’’ said Karl Simon, director of compliance in the EPA’s Office of Transportation and Air Quality.

Because mileage testing is standardized for all car makers, buyers should be confident that models with higher miles-per-gallon estimates will outperform models with lower estimates, Simon said.

But estimates for a particular model should be viewed as averages, said Dan Edmunds, director of vehicle testing for, with roughly an equal number of buyers seeing miles-per-gallon readings above or below that average.

Unfortunately, I knew none of this when I was choosing my new car. I saw the commercials and bought my Equinox thinking I’d get the advertised 29 miles per gallon. The thought that my own driving habits, or how hilly the road is to my job, could significantly affect it didn’t cross my mind.

“The car makers are trying to sell you cars, so the car makers are going to cite the highway number and talk about the highway number as if it’s fact,’’ said Edmunds.

But don’t car makers know I’ll be disappointed if I don’t get the mileage I thought I’d be getting?

“This is one of those deals where one side of the building is worried about sales and the other side of the building is worried about customer satisfaction, and the two sides are kind of at odds,’’ Edmunds said. “One of the questions on the J.D. Power survey that almost every carmaker takes a hit on is that most people are disappointed with the fuel economy that they get.’’

More accurate Despite limitations, today’s miles-per-gallon estimates are nevertheless far more accurate than ever. The EPA began generating estimates in 1974, but testing was unreliable: Estimates could overstate actual miles per gallon by 25 percent or more, experts said. The government adjusted estimates downward in 1984, but as it continued to use antiquated city and highways tests, estimates were still in large part guesswork, Edmunds said.

Finally, in 2008, the EPA revamped its testing system to factor in a number of the variables we’ve mentioned that affect fuel economy, including heat, cold, fast speeds, quick breaking, and aggressive acceleration.

The tests are still limited: Just one car per model “family’’ is tested, on a laboratory treadmill, for a total distance of about 44 miles.

The EPA also relies heavily on manufacturers to conduct the tests and submit data, randomly testing just 15 percent of vehicle model lines at its own facility in Ann Arbor, Mich., to ensure the manufacturers are accurate.

But overall, the EPA’s new system produces better estimates, particularly the overall or “combined’’ mileage estimate. The EPA’s overall estimate for my Equinox is 23 miles per gallon, which, it turns out, is exactly what I’m getting.

Although I’d like to blame carmakers and the EPA for my subpar highway mileage, alas, my own inefficient driving habits play a role, too. To improve my mileage, experts said, I need to adopt a host of fuel-saving tips. That challenge, though, we’ll save for next time.

Peter DeMarco lives in Somerville and can be reached at He also updates a Facebook page, “WhotaughtYOUtodrive?’’