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Understanding Fuel Economy Ratings

    What do those numbers on the window sticker mean and how do they arrive at them?
» Understanding Fuel Economy Ratings
» Factors That Affect Fuel Economy
» More Factors That Affect Fuel Economy
» How The Numbers Are Determined

Understanding Fuel Economy Ratings...

Recently, I had a long-time customer come in with her new car, complaining about her gas mileage. It was a 2000 model year car with about 9,000 miles on it. She said that the window sticker said the gas mileage was 27 miles per gallon in the city and 34 on the highway. She said that the best she ever got with it was 19 mpg. She wanted to know if there was something wrong with her car and whether she should bring it back to the dealer to have it looked at.

Like many other people, she misunderstood what those numbers actually mean. It's not only important to have information; it's just as important to understand what that information means. In this case, the numbers are not going to tell you what your gas mileage will be; it is just an indication of the relative fuel economy of the vehicle.

Okay, so what do those numbers on the window sticker mean and how do they arrive at them?

EPA fuel economy estimates are posted on the fuel economy label of all new vehicles. The only intended use of these values is for comparison among the different vehicles. Fuel economy estimates are generated from data taken during a laboratory test using pre-production prototype vehicles under extremely controlled conditions using a professional driver, with the vehicle operating on an instrument similar to a treadmill. The comparison of current vehicle fuel economy to the EPA fuel economy estimates is to let you compare apples to apples.

The EPA GAS MILEAGE GUIDE, available at each dealership, points out that the actual mileage when driving a vehicle may differ considerably from the estimated mileage. The guide also describes how vehicles are tested under identical conditions to ensure that the results can be compared with confidence.

The EPA GAS MILEAGE GUIDE also points out that the city fuel economy estimate simulates a 7.5 mile stop-and-go trip with an average speed of 20 mph. The trip takes 23 minutes and has 18 stops. About 18 percent of the time is spent idling, as in waiting at traffic lights or in rush hour traffic. Two kinds of engine starts are used: the cold start, which is similar to starting a car in the morning after it has been parked all night, and the hot start, similar to restarting a vehicle after it has been warmed up, driven and stopped for a short time.

The test to determine the highway fuel economy estimate represents a mixture of "non-city" driving. Segments corresponding to different kinds of rural roads and interstate highways are included. The test simulates a 10-mile trip and averages 48 mph. The test is run from a hot start and has little idling time and no stops.

The EPA GAS MILEAGE GUIDE explains that the actual test results are adjusted downward to arrive at the estimates used in the booklet and on the labels. City estimates are lowered by 10 percent and the highway estimate by 22 percent from the laboratory test results. The guide also points out that traveling at higher speeds lowers fuel economy and traveling at 65 mph instead of 55 mph lowers fuel economy over 15 percent.

Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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