The drag force could be model with F=-k1*V*A or F=F=-k2*V[sup]2[/sup]*A
Where k1,k2 are constant, A is the effective area (cross section) of the car,
and V is the relative velocity between car and wind. V=Vcar - Vwind (component in the direction of the car)
For example: if the car is moving with speed v1 to the east, and wind is v2 from east to west.
If there is a an angle c (between wind and car) V=v1+v2*cos(c);
The power loss due to the wind can be calculated as F*V
If you can find out how much power generated from the car was used to moving the car forward (much of the energy was wasted as thermal energy produced). You need to subtract the above power loss.
Another way to model it is from the experimental data. If you can record data for different conditions systematically,
model can be created from those data and we can create simulation to compare with the data.