EPA Estimates…How DOES the EPA determine fuel mileage numbers? Many people place a great amount of trust and stock in the mileage numbers on your automobile’s sticker, especially these days when money is tight and times are tough all over. But are those numbers useful, accurate, or even trustworthy? It turns out not so much, actually. Why? Well, those figures are determined in a lab using ―different than typical‖ driving conditions, rather than actual real world driving situations.
How does the EPA determine the mileage numbers? In the testing lab, the team places the drive wheels of the automobile on rollers of a machine called a dynamometer; the ―dyno‖ is designed to simulate the driving environment, much like an exercise bike simulates cycling. The machine is calibrated such that the analysts can adjust the rollers to simulate different driving factors such as wind resistance or the vehicle’s weight. While the wheels are on the dyno, a driver will run through what they consider to be a standardized driving routing (or ―schedule‖, each ―schedule‖ specifying the speed the vehicle must travel during each second of the test), which simulates a typical trip on the highway or in the city. For carbon-based fuel vehicles (i.e. gasoline, diesel, natural gas, etc), a hose is connected to the tailpipe to college engine exhaust put out during the tests. The amount of carbon in the exhaust determines the amount of fuel burned during the test (this measurement is considered more accurate than using a fuel gauge). Why hasn’t this been particularly accurate? Well, up until 2008 the speeds at which the cars were driven to simulate real world traffic were, in reality, a bit…slow. For city tests, they used an average of 20 mph, and for highway, an average of 48 mph (not sure about you, but my speedometer has never seen those numbers in those areas – oh wait, probably shouldn’t admit that). Now, after 2008, they got smart and have changed their simulation parameters to include more factors that affect mileage: more realistic driving speeds, rabbit-like acceleration, temperature variations, and even including running the A/C. Today, even though the testing process is more comprehensive, you shouldn’t expect those numbers to be exact and accurate, but they should be fairly close.