To perform coastdown tests on heavy-duty trucks, both long acceleration and coasting distances are required. It is very difficult to find long flat stretches of road to conduct these tests; for a Class 8 truck loaded to 80,000 lb, about 7 miles of road is needed to complete the coastdown tests. In the present study, a method for obtaining coastdown coefficients from data taken on a road of variable grade is presented. To this end, a computer code was written to provide a fast solution for the coastdown coefficients. Class 7 and Class 8 trucks were tested with three different weight configurations: empty, “cubed-out” (fully loaded but with a payload of moderate density), and “weighed-out” (loaded to the maximum permissible weight). To validate the method used to extract coastdown coefficients for tests on a roadway with variable grade, tests were also performed using a 2008 Ford F-150 light-duty pickup for which the present coastdown coefficients were compared to those available from the United States Environmental Protection Agency.Additionally, the effect of the coastdown coefficients on the fuel consumption of Class 7 and Class 8 trucks is discussed, where the coastdown coefficients were among the input parameters to the University of Texas Fuel Economy Model. This vehicle simulation software was developed for the Texas Department of Transportation to estimate the fuel consumption of both light-duty and heavy-duty vehicles.