Vehicle road load which must be simulated on a chassis dynamometer for exhaust emission and fuel economy tests is usually determined by on-road coastdown tests. In this study, we divided vehicle road load into rolling resistance and aerodynamic drag, and measured these forces in laboratory tests, using a high-precision chassis dynamometer and a full scale wind tunnel. However two problems arise. One of them is that tire loss measured on the curved drum of a chassis dynamometer is larger than on the road. Another is that aerodynamic drag measured in a closed-throat wind tunnel, which has blockage effects, is larger than on the road. To determine vehicle road load from laboratory tests, we therefore have to establish how to obtain a correction coefficient for tire loss between the actual road and the drum, and a correction coefficient for aerodynamic drag between the actual road and a wind tunnel. How we get these coefficients is described in this paper.