

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.)

















It seems like what you’re actually asking is how far will it be able to coast assuming a starting speed of 100 MPH.
That will be hugely dependent on tons of variables, not least of which being rolling resistance of the tires on the pavement (itself affected not just by the tires but also temperature, and moisture, and diameter of the tires, and roughness of the asphalt…) plus a car’s drivetrain is chock-a-block full of frictional losses throughout oodles of bearings and the constant rub of the brake pads on the rotors even when the pedal isn’t depressed, and so on and so forth. So I don’t think anyone will be able to accurately calculate that for any particular car. A field test is probably your best bet. If you’d like to not get arrested, start from 50 MPH instead and double your result. I conjecture that the deceleration from road friction and other losses will be near as makes to difference to linear.
A car rolling in neutral on a flat surface can coast pretty far. With a lifetime of driving experience behind me, my gut feeling says a quarter mile is definitely possible, and half a mile probably is as well. Coasting a full mile feels like a bit of a stretch.