When considering aerodynamics of drag racing cars/trucks, etc. The assumption of aerodynamic drag only has an effect at speed is incorrect.
One can think of the issue becoming important at any speed over 40mph and the aero drag resistance is exponential to the speed very closely following the square law and more. That becomes a lot of free power by cleaning up (legally of course) the aerodynamic drag for the vehicle.
Lots of rules of aero thumb to apply, but it is best to study and learn what can help with the selection of vehicles and how to improve them. An example is: external rear view mirrors cost about 0.6% in drag power.
Regards,
HB2
Dissident