September 21, 2005

How government fuel economy ratings are determined
by Jim Kerr

So you're ready go new car shopping. You have selected several vehicles that meet your needs, but fuel economy is important too. The fuel economy numbers look good on the window sticker, but are they really accurate? How do they test them?

Canada and the U.S. use similar Federal Test Procedures (FTP) for testing the fuel economy of new vehicles. Each test is a standardized laboratory test method where a trained driver drives the vehicles on a dynamometer for a predetermined drive cycle. The test is structured so that each vehicle is driven under identical conditions and that the results for each vehicle are consistent.

Two different tests are used. One tests fuel economy for city driving and the other for highway driving. Actually, the FTP tests are used to certify the vehicles to emission standards, but the fuel economy can be calculated by measuring the carbon compounds emitted during the test.

The fuel economy estimates are printed on the new vehicle window sticker as well as the annual estimated cost of fuel the vehicle would use if driven a distance of 20,000 km with 55% city driving and 45% highway driving. Of course, with the recent surge in fuel prices, those annual fuel costs on the window stickers are now long out of date and it could be some time before things stabilize and the data is close again.

The Canadian FTP city test simulates a 12-kilometre stop-and-go drive lasting 23 minutes. The drive begins after starting a cold engine, to simulate a vehicle parked overnight. It includes 18 stops, four minutes of idling time, a maximum speed of 91 kph and an average speed of 32 kph. When the test is completed, the first 8 minutes of the test are repeated starting with a hot engine to simulate a vehicle that has been stopped momentarily and then restarted.

The U.S. city test simulates an 11-mile (17.7 km) stop-and-go drive lasting 31 minutes. Like the Canadian test, the engine is started cold. It includes 23 stops with 18% (about 5 ½ minutes) of the drive spent idling. Maximum vehicle speed is 56 mph (90 kph) and an average speed of 20 mph (32 kph). Tests are done between 68 F and 86 F (20C to 30 C) ambient temperature.

The Canadian and U.S highway tests appear to follow a similar format. It is a 16-kilometre drive that lasts 13 minutes. This test starts with a hot engine and does not include any stops but speeds are varied to simulate both highway and rural roads. The average driving speed is 77 kph, with a top speed of 97 kph (60 mph in the U.S.). Both city and highway tests are performed following a preset pattern so that results can be duplicated.

Although the Canadian and U.S. tests are very close, converting a U.S. miles per gallon fuel economy rating to Canadian litres/100 km rating will not give you the same figures as the same car tested in Canada. It is best to compare U.S. ratings to other U.S. vehicles and Canadian ratings to other Canadian vehicles.

It is recognised that these test results do not represent actual driving economy. They are done on a dynamometer so there is no wind resistance. The tests are also done at warm ambient temperatures. The test results are factored for each model of vehicle to provide more accurate results. In the U.S., the city estimate is lowered by 10% and the highway estimate is lowered by 22% from the laboratory results.

The auto manufacturers do the initial fuel economy and emission testing for each vehicle. The government doesn't have the resources to test all vehicles, so they rely on the data provided to them. To ensure accuracy, Transport Canada and Environment Canada purchase a specific number of new vehicles each year that have high production numbers, new engine technology or have had problems in previous years. These vehicles are tested and compared to the auto manufacturer's data.

If your vehicle doesn't get as good fuel economy as the ratings, it could be because you are driving it under different conditions. Short trips, idling, cold weather and high speeds all use more fuel. 4WD trucks are tested in 2WD mode if possible, so operating in 4wd will use more fuel than the rating. Typically, summer gasoline blends also contain more BTU energy than winter blends, so there is also a slight decrease in fuel economy in winter just because of the fuel.

Don't expect to get the same fuel economy as is printed on the window sticker. It is a good tool to compare one vehicle to another when shopping. Drive carefully and you may be close to the ratings - some actually get better economy! In my experience however, most drivers get a little lower fuel economy that the window sticker.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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