How the 2008 EPA Fuel Economy Ratings Work

By: Jamie Page Deaton

The fuel economy ratings system has changed since this man went shopping for a Chevy.
The fuel economy ratings system has changed since this man went shopping for a Chevy.
Tim Boyle/Getty Images

When you're shopping for a car, there are a lot of numbers to consider on the window sticker. Most people look at the car's price first, then let their eyes wander over to its horsepower and torque numbers. But with

gas prices hitting $4 a gallon in some areas,­


a lot more eyes are focusing on the car's fuel economy rating.

Fuel economy is a measure of how many miles a car can travel using a gallon of gasoline. Knowing a car's fuel economy is helpful to car buyers because it allows them to estimate the actual cost of ownership for a car. It's one thing if a car has a low sticker price, but week after week of paying lots of money just to keep the car rolling wears on most drivers. A car's fuel economy is measured by the federal government in a program run jointly by the Department of Energy and the Environmental Protection Agency (EPA).

The government measures fuel economy not only to help out consumers, but also to make sure that car companies are acting in the country's best interest. Having too many inefficient cars on the road is bad for the environment and the economy, since so much oil has to be imported.

The government has been estimating fuel economy on all cars and light trucks on U.S. roads since the 1960s, but for 2008, it revised its estimates. Why did the government make the change, and how will it affect you? Read on to find out.­



Fuel Economy and the Government

People might be confused by a surprising increase in fuel efficiency between 2007 and 2008.
People might be confused by a surprising increase in fuel efficiency between 2007 and 2008.
JGI/Blend Images/Getty Images

At first, it seems a little odd that the government would measure fuel economy, let alone have two governmental agencies spend time on it. After all, if consumers cared about fuel economy, automakers would test it themselves and advertise the results to gain customers, right? Well, almost. Auto manufacturers do perform their own fuel economy tests, but since it's in their best interests to get the good numbers, each company could perform the tests or manipulate the data in ways that benefit them. The government does the same test on all cars, making its numbers impartial.

The government also has an interest in monitoring the fuel economy of all the cars on the road. Improved fuel economy means less pollution and less dependence on foreign oil. That's why both the Environmental Protection Agency (EPA) and the Department of Energy are involved in testing fuel economy ratings, though the EPA takes a lead role in testing and publicizing the ratings.


Fuel economy ratings have been given out by the government since the 1960s. The problem was that the tests and the ways the ratings were calculated hadn't really changed much since then, despite the fact that cars had changed a lot. Cars can accelerate faster now and have more accessories (like air conditioners) that use extra gas. Highway speed limits are also higher now, and driving faster takes extra fuel. The government changed how it tests fuel economy to take these factors into account.

How did the government change the test to reflect modern-day driving? Read on to find out.


Comparing the Fuel Economy Tests

It was time for fuel efficiency tests to get a tune-up.
It was time for fuel efficiency tests to get a tune-up.
Mervyn Penrose Rands/Getty Images

The original test used to estimate a car's fuel economy was based on everyday driving in the 1960s, and the last update to the test was in 1985. That means it was geared toward slower acceleration, slower highway speeds, and few in-car accessories that draw power from the engine. The test made a lot of sense at the time, but as cars have changed, the test needed to as well.

The test itself is performed on a dynamometer, which allows testers to control the testing environment and get accurate readouts because the car can be attached to sensors. The original test on the dynamometer mimicked everyday driving by taking the car through simulated stop-and-go traffic and highway driving. The test results were then multiplied by a set number, to account for things like wind resistance.­


The new test is very similar to the old test. It still takes place on a dynamometer and still multiplies the results by a set number to take wind resistance into account and get the final fuel economy estimate. What's changed are the driving conditions that the dynamometer mimics. Now the test uses faster acceleration and highway speeds. It also accounts for the use of air conditioning. Finally, the test mimics a start in 20 degree cold (the colder the weather, the more gas it takes to start a car). With these changes, the new test now more closely resembles the type of driving most people do.

So now you know how the test has changed, but what does it mean for you? We'll explain that on the next page.


The Results of the 2008 Fuel Economy Ratings

Efficient cars like the Toyota Prius didn't show as big a drop in fuel efficiency ratings as larger automobiles.
Efficient cars like the Toyota Prius didn't show as big a drop in fuel efficiency ratings as larger automobiles.
Justin Sullivan/Getty Images

It's a common complaint drivers have: Their fuel economy doesn't come close to fuel economy that was promised on the car's window sticker. In the past, that was due largely to the fact that the ratings were developed in the 1960s. With the new 2008 fuel economy ratings, drivers should expect to get fuel economy that more closely matches the fuel economy numbers given by the government.

The results can be a bit more confusing than that. Overall, fuel economy numbers have declined because of the new ratings. The 2007 Toyota Camry, for example, got 24 mpg in the city and 33 mpg on the highway, for a combined fuel economy of 27 mpg under the old ratings. With the new ratings, the same car gets an estimated 21 mpg in the city and 30 mpg on the highway, for a combined fuel economy rating of 24 mpg. That change in fuel economy for the same car can be confusing for car shoppers, but it's not the result of any changes to the car. It's because of the changes to the test. Overall, consumers can expect to see city fuel economy ratings for all cars drop about 12 percent, with highway ratings dropping about 8 percent [source:]. Some cars could show a drop of as much as 30 percent in their city mileage rating and 25 percent in their highway mileage rating [source:].


While all new cars are now following the new test and have the new ratings, the change can make shopping for a used car more challenging, because it can be tough to tell if the fuel economy rating on the sticker is based on the old or new test. If you're buying used, the best way to check is to visit, where the government has posted the old and new fuel economy numbers for all mainstream car models, going back to 1985.

­­To learn more about the EPA and fuel economy, look over the links on the next page.­


Lots more Information

Related HowStuffWorks Articles

More Great Links

  • Edmunds, Dan. "Explained: 2008 EPA Fuel Economy Ratings"
  • Environmental Protection Agency
  • Environmental Protection Agency. New Fuel Economy Ratings FAQs
  • Healy, James R. "Car mpg Ratings Going Down." March 5, 2007. USA Today.
  • Jerome, Marty. "Fed Poised to Accelerate CAFE Standard." April 22, 2008.
  • Vazquez, Yolanda. "New EPA Fuel Economy Ratings." MotorWeek
  • U.S. News Rankings and Reviews. "Auto Sales Crashed in March." April 2, 2008.