Rather than doing the sensing on a wheel, it would probably be better to do it
on the drive shaft if using a rear wheel drive vehicle. this would then average
the difference between the inner wheel and outer wheel when turning corners. If
the car did a lot of say left hand corners compared to right hand corners
(especially in city driving) you may find a significant error creeping into your
calculation.

If the vehicle is front wheel drive, then it becomes a lot harder. The ultimate
would be to attach to the speedo takeoff like already mentioned.