reggietank69 wrote:
what would it be if i was running 31'' tyres
It pretty much depends on the tyre. No tyre runs exactly to 31" no matter what anyone tells you.
But maybe have a play around here:
http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx, and use a GPS (or correct your gearing).
(Disclaimer: Obviously if you buy a tyre just over 31", at some point in its wear cycle it'll be exactly 31" in diameter. Don't be a dick; you know what I am saying.)