Speedo Accuracy

[scillyisles]

I think most of us with standard 7x17" wheels are now running 225/45ZR17 tyres. I've been doing measurements on speedo accuracy using a couple of portable GPS Sat Nav systems and discovered that unlike most cars my RS2 speedo is more or less spot on.
This is unusual in that car speedos normally overread and I think one of the causes is the change from OEM 245/40ZR17 Dunlop SP8000s to my previous Bridgestone 225/45ZR17 RE050 and now Pirelli 225/45ZR17 Neros.
The OEM tyres weere really only as wide as most normal 225s and assuming they were 40 section this gives a wheel diameter with tyre of
(225*0.4)x2)+ (17x25.4) = 611.8. Whereas a normal 225 tyre of 45 profile gives a wheel diameter of ((225x0.45)x2)+(17x25.4) = 634.3.
Overall, I think this leads to a 3.6% speedo underreading probably meaning that your speedo is more acurrate than on the OEM tyres.