I've noticed amateur road vehicle testers (don't know any professional road vehicle testers) use coefficient of drag (Cd) multiplied by the frontal area of the vehicle (A) to characterize the drag of their vehicle.
This method gives you two variables that must be obtained to determine your vehicle drag.
I'd like to suggest a method used in the Flight Test world called Equivalent Flat Plate Drag (EFPD). When we modify our aircraft, we test and note the change in the flat plate drag area (FPA). This allows us to determine performance changes very simply.
To use this method, assume a Cd of 1.0 and allow the drag calculation to determine the "Frontal Area" (A) for you. This frontal area will not physically represent the frontal area of the vehicle but it will allow the user to have an equivalent representation for vehicle comparison.
Example:
(...all numbers are fictional...)
LARGE TRUCK - Drag = 6250 Newtons on a day with density of 1.00 kg/m^3 (for simplicity of calcs) @ 25 m/s.
(FPA)*.5*density*velocity^2 = 6250
FPA*.5*1.00*(25^2) = 6250
FPA*.5*1.00 = 10
FPA = 20 m^2
....
A motorcycle might have numbers similarly to 5 m^2
Having these numbers allows someone to see how the fuel consumption on the truck should be more than the motorcycle based on the aerodynamics of the vehicle alone. If you could modify the vehicles to have the same FPA, the fuel consumption difference will be resultant from a different source.
Happy Modding!
Ryan