[meanjoe75fan]

1. Since rr is assumed to be constant W/R/T velocity, what about pushing/pulling car, on a level paved surface, with a scale to show force? (Ex: rope tied to bumper, to scale, rope to puller). Note that the "breakaway" force to get the car rolling WILL be higher than that to KEEP it rolling...the latter is what you want.

2. The "coastdown" method requires a. a flat road for >= 1mi and b. no traffic/police presence. Since these (esp. a.) are infrequent where I am, what about terminal velocity on a gradient of known %? The benefit is both that roads are engineered to have constant grades, and that the 4-6% highway standards will produce quasi-legal terminal velocities for most cars...drawing a lot less attention.

3. Using rho (~1.22 in SI units): Air pressure is 29.92 " Hg at S/L and drops about 1"/1,000 ft. This rule of thumb is good to at least 8,000'...so, 99.44% of motoring roads. So, find rho for your temp, and knock off 1/30th for each 1,000' above S/L.

EDIT: Just wanted to add I gave this a try...just a 0.3 mi stretch with constant (slight) grade. T.V. in four runs was: 66, 65, 64, 65. I also verified 60 was too low (+1.5 MPH) and 70 was too high (-2.0 MPH). Worked fine--very consistent!

Now all I need to do is get out there with a carpenter level and calculate grade, and I'll know how many # thrust it takes to maintain 65 (corrected for wind...)