Turns out that coastdown testing calculation is not easy!
Right now I am looking at using
this Instructable as a start, but with using a starting speed of 115 kph and an ending speed of 15 kph instead.
I remember the discussion about there actually existing a V term that corresponded to drivetrain-related viscous drag, so there's a complication there. Not really sure how I'd calculate a C(d) either, since people have pointed out that cars have a negative lift component at speed which is going to throw off the V^2 term calculation. Hm.. at least C(rr) could still be reliably calculated... perhaps.
I think the best to be hoped for is to just do a sort of stopwatch function that starts when the vehicle slows down and passes 115 KPH, and stops when the vehicle reaches 15 kph. One could do an average of several passes in that fashion, and store the resultant elapsed time for A-B-A comparisons.