The number that I used in my Quarter2.xls program should not be taken as gospel. It is a number that I arrived at by analyzing the coast down graph published in the 1970 review of the 240Z by Car and Driver.
What I did was pick two point on the coast down graph (80 mph and 20 mph) and estimated the slope of the curve at those two points. The slope of the coast down curve at any point corresponds to dV/dt. Using the instantaneous accelerations at those two points, I solved the following :
m*dV/dt=CdA/2*Rho*V^2 + Cr*V + C
In this equation dV/dt was estimated from the graph at two points.
V is the velocity at the two points chosen.
Rho is the density of air
C is a constant value that I used to account for the force needed to start the car moving from rest on a flat surface.
The variables CdA and Cr are the drag coefficient times frontal area and the static rolling resistance. The units of CdA are ft^2 and the units of Cr are lbf/mph.
One factor that I neglected in my original analysis was the inertia of the wheels and tires. To account for this the inertia of an original equipment tire, wheel and brake assembly would have to be measured and the equations would need need an additional term.
m*dV/dt + I* dw/dt = CdA/2*Rho*V^2 + Cr*V + C
where the term I x dw/dt is the mass moment of inertia of the rotating parts times the change in rotational velocity.
Although I was trying to be as accurate as possible, it should be remembered that I made the Quarter2.xls program for fun. Even neglecting the inertia of the rotating assemblies, I believe the number to be accurate within +/- 15%.
As stated on the downloads page, I welcome input to increase the program's usefulness and accuracy.
Dan