So address the fact that there are 4000 lb boxes doing 70 mph. Physics, to a first order approx, doesn't care about the car mass, btw. It does care about A*Cd

So address the fact that there are 4000 lb boxes doing 70 mph. Physics, to a first order approx, doesn't care about the car mass, btw. It does care about A*Cd*v^3 though. Especially the v. A lot of problems depends on that v.