In automobile development, wind tunnel measurements are used to optimize fuel consumption and the vehicle's road behavior. The classic measuring technique is based on a stationary vehicle set up in the wind tunnel with stationary wheels. Relative movement between vehicle and road surface is therefore ignored. In more recent studies, measurements have been taken with improved ground simulation. For example, a belt is used instead of the stationary wind tunnel floor and the car wheels rotate.Ground simulation using a belt and rotating wheels generally leads to a reduction in flow angularity at the front wheels, in the same way as blocking the cooling air flow, whereby, as a matter of fact, the aerodynamic drag is reduced. Analogous air flow angle correlations can be established for the effect of underfloor panels. Thus, ground simulation affects local flow quantities as well as total air flow values, not only with regard to the absolute drag coefficient, but also to differential measurements governed by the configuration. The classic measuring technique in certain cases, leads to a change in the sign of ΔcD when assessing optimization measures.In conclusion, correlations between reductions in drag and fuel consumption are established. The rule of thumb is that, in the case of optimization measures which have such a minimal influence on weight that the car's inertial mass category remains the same, a reduction in drag, e.g. by 1 % ΔcD≈0.003), has the same effect on fuel consumption in the European MVEG II cycle as an equally large reduction in weight (10-15 kg).