For many car shoppers, fuel economy is one of the most important reasons to buy a car. As a result, many car buyers pay close attention to Environmental Protection Agency (EPA) gas-mileage ratings, which are shown on the window sticker of every new car sold in the United States.

But owners sometimes discover that the gas-mileage numbers on the window sticker aren't the same as their own real-life figures. Has this happened to you? We have some answers concerning why you aren't getting the mileage you were hoping for.

How EPA Tests Cars

It may surprise you to learn that EPA doesn't test cars for fuel economy. That's because EPA doesn't have the time or the budget to test all the new cars offered for sale in the U.S., especially when you consider the wide range of engine, transmission and drivetrain combinations available.

As a result, EPA provides the testing procedure to automakers, and automakers are entrusted to carry out the tests on their own. Of course, this can lead to problems, since automakers want fuel economy numbers to be as high as possible. An example of this came in 2012, when Hyundai had to redo fuel economy figures for more than a dozen cars after EPA received hundreds of complaints that the automaker had been publishing gas-mileage figures that were higher than they should've been.

Automakers obviously have an incentive to overstate the figures, which is a key reason why you might not see the gas-mileage ratings you were hoping for. While we think that such overstating is rare, there's no doubt that it happens occasionally. Fortunately, there are other websites -- including Fuelly and a My MPG section of EPA's own site -- that allow users to submit their own fuel economy data so they can track their miles per gallon.

Driving-Style Differences

Another good reason you aren't getting EPA-estimated fuel economy figures is that your driving style doesn't match EPA's. This primarily relates to shoppers who mostly drive in the city, those who use lots of vehicle accessories, or anyone who lives in an area with lots of steep hills.

Here's what we mean: While most drivers take EPA's combined rating as an estimate of what gas mileage they should be getting, most drivers aren't driving on EPA's combined cycle. EPA's combined driving cycle assumes 55 percent city driving and 45 percent highway driving, figures rarely attained by those who spend all their time in the city. Also, remember that just because you're on the highway doesn't mean you're engaged in highway driving for fuel economy purposes. Bumper-to-bumper traffic on the interstate may take place on the highway, but it's a lot more like city driving.

Another aspect that can affect your fuel economy is the constant use of vehicle accessories. For example, if you're always blasting your air conditioning, you're probably using more fuel than EPA estimates, so drivers in hot climates are especially at risk for not achieving EPA fuel economy figures.

Your local terrain can also play a part in fuel economy differences. If you live in a hilly area, for example, you might find your vehicle struggling with hills, so you'll have to accelerate more to climb them. This increases engine speed and burns more fuel than if you were driving on a flat, level surface.

Other Differences

Although driving style is likely the main reason for a difference between your fuel economy figures and EPA estimates, a few other factors may also come into play. For instance, if your car is older or poorly maintained, fuel economy likely won't be the same as it was when it was new.

Interestingly, the same is true for a car that just rolled off the lot: An engine that has not yet been broken in may not reach its full fuel economy potential until it's driven a few hundred miles.

Should You Trust EPA Ratings?

In our view, EPA fuel economy ratings are an excellent guide, but they're not supposed to be taken as absolute fact. This is why EPA calls its ratings estimates; it's a good term considering all the potential variations that drivers might encounter when they're actually out on the road.

As a result, we think that EPA ratings are a great way to compare cars, but if you're looking for real-world gas mileage, then sites like Fuelly may provide extra help. Some dealers will even let you take a car on an extended test drive, occasionally overnight, so you can measure gas mileage yourself.

author photo

Doug DeMuro has a wide range of automotive industry experience, from work at a Ferrari dealership to a manager for Porsche North America. A lifelong car enthusiast, Doug's eclectic vehicle purchases include a Porsche 911 Turbo, an E63 AMG wagon, an old Range Rover and a Mercedes Benz G-wagen.

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