Chip, what do you use for the diameter of the wheel/tyre? Just (wheel dia + 2x profile) with profile being taken from the width and profile of the tyre size e.g. for 205/50 15 = 58.6 cm.
Just wondering if you account for tyre deformation or true profile rather than tyre size.
Thanks
I took a datalog from a car running the tyres so that I had a reference point to work from.
It will still vary by a couple of RPM though based on amount of tread left and pressure run, so dont take it as an EXACT set of numbers, but it should be withing a couple of mph.
For 195/50/15 it will of course be a slightly less in all cases.