I agree with your methodology and I’m further stoked to see that my assessment, which was largely a guess generated by inverting the relative speeds and masses of the body of the car and its wheels, arrived at a rather similar result. Neat.

(I used the weights from my Crosstrek, which is a bit heaver than a GT86 and mine at least has larger diameter wheels, but coincidentally uses an engine based around the same design. FA/FB gang, yo.)