A gauge would be very helpful. But coastdown testing is a relatively quick way to measure the effect of aero mods.
The caveat is: the modification needs to be significant enough to be detectable outside of the noise of normal variation in the runs. How significant? For my Firefly/Metro, I could
just detect the difference between driver's window open vs. closed in coastdown times.
http://ecomodder.com/forum/showthrea...open-8863.html