Predicting Light-Duty Vehicle Fuel Economy as a Function of Highway Speed

The fueleconomy.gov website provides information such as “window label” fuel economy for city, highway, and combined driving for all U.S.-legal light-duty vehicles from 1984 to present. The site is jointly maintained by the U.S. Department of Energy and the U.S. Environmental Protection Agency (EPA), and also offers a considerable amount of consumer information and advice pertaining to vehicle fuel economy and energy-related issues. Included with advice pertaining to driving styles and habits is information concerning the trend that as highway cruising speed is increased, fuel economy will degrade. An effort was undertaken to quantify this “conventional wisdom” through analysis of dynamometer testing results for 74 vehicles at steady-state speeds from 50 to 80 mph. Using this experimental data, several simple models were developed to predict individual vehicle fuel economy and its rate of change over the 50-80 mph speed range interval. The models presented require a minimal number of vehicle attributes. The simplest model requires only the EPA window label highway mpg value (based on the EPA-specified estimation method for 2008 and beyond). The most complex of these simple models uses vehicle coastdown test coefficients (from testing prescribed by SAE Standard J2263) known as the vehicle Target Coefficients, and the “raw” (unadjusted) fuel economy result from the federal highway test. Statistical comparisons of these models and discussions of their expected usefulness and limitations are offered.