I had made a thread about this.
http://ecomodder.com/forum/showthrea...tor-24478.html
The file is attached in the first post. Go on the Cd/Cr tab. You'll probably have much higher Cd values than reality. That's due to the various frictions (bearings, brake drag, gearbox...) that are not taken into account and since their forces are linear with speed, you can add them up to the Cr.
Example with m = 1000 kg, Cr = 0,010 and A = 2 m². 25 seconds to coast down from 120 to 80 km/h gives Cd = 0,373 but you know your car is around 0,30. Increase Cr so that Cd is approx. 0,30. In this case 0,017 (0,010 for tires and 0,007 for various frictions). Then, play with the time to see Cd reduction.
The tool is good to calculate relative Cd decrease but not the absolute value.