Estimates of level-road, constant-speed fuel savings from a tractor-trailer aerodynamic device were made using 1/7 scale model wind tunnel tests. These estimates were compared to fuel savings measured during a specific series of on-road tests of the same vehicle and aerodynamic device.Scale-model wind tunnel tests were run according to the SAE J1252 procedure. Changes in the vehicle's wind-averaged drag coefficient due to the aerodynamic device were used to compute estimates of fuel savings. Scale-model details were important in computing these estimates. On-road fuel economy tests were run per the SAE J1321 Type II procedure. Examples of the effects of ambient wind on aerodynamic device effectiveness measured during SAE J1321 tests are presented.