[MPaulHolmes]

OK, I believe it's reasonable to assume that the force from rolling resistance is constant, and the force from wind resistance is proportional to the speed of the car.

Let 'v' be the speed of the car.
Let 'F' be the force acting against the car in neutral at speed 'v'.
Let 'p' be the constant force from the rolling resistance.

F(v) = p + q*v, for some constant q.

So, choose a speed v1. Using the approach in the previous post, find F(v1).
Choose speed v2. Find F(v2). So, now you have 2 equations in 2 unknowns, where the unknowns are 'p' and 'q'. Find p and q.

Now, you can know the force acting against the car (in neutral) for any speed you want, since you now know p and q.

By the way, p is the rolling resistance. So, it would be interesting to try the experiment for various tire pressures, to see how much p changes. You could even come up with a curve for rolling resistance as a function of tire pressure. I think I'll do that.
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