TexasTesla
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How Vehicles Are Tested
Fuel economy is measured under controlled conditions in a laboratory using a series of tests specified by federal law.
Manufacturers test their own vehicles—usually pre-production prototypes—and report the results to EPA. EPA reviews the results and confirms about 15%–20% of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.
Estimating MPG with Laboratory Tests
In the laboratory, the vehicle's drive wheels are placed on a machine called a dynamometer. The "dyno" simulates the driving environment much like an exercise bike simulates cycling.
Engineers adjust the amount of energy required to move the rollers to account for wind resistance and the vehicle's weight.
Every last detail of an EPA fuel-economy test has specific rules—there’s even a set volume of air that a fan must blow under the car’s raised hood. “Driving” the test by matching the red line is tricky and takes a sensitive foot.
After a vehicle is strapped down on a dyno, the staff punches in coefficients that allow the dyno rolls to simulate real-world factors, such as wind and road friction.
One of the EPA’s drivers is behind the wheel of the test car. With an average of 20 years’ experience, they have extremely fine-tuned throttle and brake-control skills. They “drive” by following a precise red line of speed versus time that’s displayed on a monitor hanging just in front of the windshield. Using the gas pedal and the brakes, the driver attempts to match the red line with the car’s wheel speed, which is shown in white. We got into a test car and tried it, and indeed, it’s very difficult to maintain the speed of the tests, particularly when it dithers in the single digits and a brush of the throttle can send the white line careening off-course. If the speed deviates from the test cycle by more than 2 mph, the results are thrown out. For manual-transmission cars, there are standard EPA shift points, which are broadcast on the driver’s screen as well.
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