• EPA-estimated miles per gallon included on every new-car window sticker
  • Official economy numbers allow comparison between models
  • Real-world economy affected by factors like driving style and weather

When shopping for a new or used car, one consideration that can have a significant impact on the cost of vehicle ownership is fuel economy. Fortunately, the Environmental Protection Agency (EPA) provides ratings on every new car, allowing shoppers to assess fuel costs before they buy. But the official ratings may not be accurate for every driver. So how can EPA ratings really help shoppers make the best decision?

The first thing you need to know about the EPA fuel economy numbers is that they're estimates. The numbers printed on a vehicle's window sticker are not the exact numbers you'll achieve once you buy that new car. Even though a driver's real-world fuel economy varies based on driving style, weather, terrain and road conditions like heavy traffic, the EPA's official numbers can provide an apples-to-apples comparison between new vehicles. For example, the 2013 Honda Accord with a four-cylinder engine is rated at 29 mpg in combined city and highway driving. The Honda CR-V is rated at 26 mpg combined. A driver who spends more time in stop and go traffic may achieve slightly lower actual miles per gallon in both vehicles. But that driver would still likely see a difference of about 3 mpg between the Accord and the CR-V.

The official EPA fuel efficiency information on a new-car window sticker also provides an estimate of costs, so shoppers don't have to calculate the difference in dollars on their own. Costs are based on average fuel prices and 15,000 miles per year, with 45 percent highway driving and 55 percent city driving. Comparing the Accord and CR-V again, the EPA estimates that the Accord costs $1,800 in fuel per year, while the CR-V costs $2,000, a difference of only $200 per year. And those costs compared across different types of cars may not be as radical as you think. A 2013 Toyota Corolla with an automatic transmission will cost about $1,800 per year according to the EPA. A much larger SUV with three rows of seats like the two-wheel drive 2013 Toyota Highlander with a four-cylinder engine will cost the average owner $2,400 per year, a difference of $600 per year or about $50 per month. Those who drive more than 15,000 miles per year will likely save more money on fuel when choosing a more efficient vehicle, so be sure to consider how your driving habits differ from the assumptions used in the official numbers.

Because the EPA does not test every new car, the agency relies on automakers to provide estimates. The official test is a carefully controlled simulation of real-world driving conducted in a laboratory, not a driving test conducted on actual roadways. The lab test levels the playing field by eliminating factors such as terrain and weather that could make some vehicles look more or less efficient than they actually are. Because the test is uniform, the results can be compared across models. To ensure accuracy, the EPA verifies about 10 to 15 percent of these tests with their own analysis.

Changes to the EPA test for the 2008 model year make the results from previous years incompatible. Shoppers considering both new and used cars should only compare ratings for 2008 and later vehicles with new models.

Fuel efficiency estimates for every new vehicle are listed on the official federal website Fueleconomy.gov. The site allows shoppers to compare vehicles side by side, and even allows them to submit their real-world results.

Of course, drivers will rarely achieve the exact miles per gallon reflected by either the official EPA ratings or the estimates from owners on Fueleconomy.gov, but both provide a good baseline for comparison when shopping. And while factors like heavy traffic can bring down real-world results, we've occasionally done better than the official numbers by driving a little more conservatively.

What it means to you: Drivers are unlikely to achieve the exact fuel efficiency estimated by the EPA, but the official numbers rely on uniform testing that simplifies the comparison of different models.

author photo

Nick Palermo is an automotive writer and lifelong car nut. He follows new and late-model used vehicles for AutoTrader.com, writes about vintage cars for Hemmings Classic Wheels and blogs on all things automotive at LivingVroom. He lives in Atlanta with his wife and twins.

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