How we test: Fuel economy

Consumer Reports News: July 24, 2006 10:14 AM

We’re often asked how we get our fuel economy numbers for our tested cars and why they’re different from government estimates. These are natural questions, since the EPA fuel economy numbers are often much different than those published by Consumer Reports. Simply put, the government tests fuel economy differently than we do.

Our fuel economy tests are performed with calibrated equipment to measure the exact amount of fuel used in different driving tests. We do not rely on trip computers, odometers, or service station fuel pump meters.

When we test for fuel economy, two drivers are used for each car tested, one driver to a car, and tested independently.

Using a very accurate fuel flow meter, we measure the actual amount fuel used in three strictly controlled tests. A city cycle test is conducted on our test track to ensure every car is tested identically. A highway test is conducted at 65 mph on a five-mile section of road run in both directions, to limit the effects of wind and grade change and a one-day trip test of mixed driving. This test includes highway, major roads, and some stop-and-go driving. We require that the temperature must be over 32-degrees F and the road surface must be dry prior to all fuel economy testing. We use an SAE fuel temp correction factor for all our mpg tests to correct for temperature differences throughout the year. The wind speed should also be no greater than 10 mph.

The government uses calculations from the vehicle emissions certification test. These tests are conducted in a laboratory where the car is driven on a dynamometer, which is a bit like a treadmill for cars. (Four- and all-wheel drive vehicles must be driven in only two-wheel drive mode or modified to do so.) Cars are put through a driving cycle that is based on driving styles, speeds, and traffic conditions of the 1970s. The amount of carbon in the exhaust is measured to calculate the amount of fuel used during the test. Over the years vehicle manufacturers have learned ways to optimize their vehicles and test conditions to obtain higher fuel economy ratings. We feel our fuel economy numbers are closer to what consumers are likely to see with the vehicle, as they are measured from actual on-road tests and exact fuel used.

We calculate the cruising range figures from our one-day trip mileage (not rounded off), which is always higher than the "overall" figure. We multiply the one-day trip mpg by the tank size in gallons minus a 30-mile "safety zone," rounded to the nearest five miles, to determine the cruising range.

CR's overall mileage and the cost of fuel used in 15,000 miles are calculated from equal portions of city and expressway driving and from the one-day trip mpg.

To learn more about how CR and the EPA test fuel economy, and how to improve your gas mileage, visit A Guide to Stretching Your Fuel Dollars.

--Mike Quincy

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