Fuel Economy Measures
The EPA currently employs two tests in the calculation of new vehicle fuel economy for use in determining a manufacturer’s compliance with CAFE standards. The tests, conducted in laboratories on chassis dynamometers, are designed to reflect a representative city driving schedule and highway driving schedule. The results are harmonically averaged, 55 percent city and 45 percent highway, to calculate a vehicle’s fuel economy test value. After the end of a model year, vehicle manufacturers provide sales data to the Federal Government, and the manufacturer’s CAFE is determined by harmonically averaging the tested vehicle fuel
economies, weighted by their respective sales shares.
It is an accepted fact that tested fuel economy values used to determine CAFE compliance are seldom achieved during actual on-road driving conditions. The difference can be attributed to driver behavior, weather, traffic conditions, terrain, etc. As a result, the fuel economy values provided to consumers on vehicle window stickers and published in the Gas Mileage Guide are adjusted to represent more accurately the fuel economy consumers are likely to achieve during ownership. This adjustment reduces the tested city fuel economy by 10 percent and the tested highway fuel economy by 22 percent, which lowers the average tested fuel economy by about 15 percent. Studies have shown that continual increases in the share of highway travel, average highway travel speeds, and congestion result in on-road fuel economy values that are approximately 20 percent below the tested values.
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