[aerohead]

Coastdown measurements are the only other option for ascertaining drag coefficient short of wind tunnel testing.
The technical requirements for SAE-approved (scientific) coastdown tests are virtually beyond the scope of do-it-yourself.
If you seriously want to pursue coastdowns,get a hold of the SAE Handbook (4-volumes plus Index) and you'll find their testing protocol .It's 3-pages of differential equations solving simultaneously for a handful of unknowns unless you can absolutely control your testing environment.
I'll let you do the research and make your own decision.I can tell you that what you're doing will never produce useful quanta.