I've had a quick search and this "app" for android doesn't seem to have been mentioned.
My brother (RoadkillUK) pointed it out to me, I've been using it and it's spot on for working out tyre sizes/circumferences etc.
Free too

Tyre Size Calculator on android market



[Edited on 8/6/11 by UncleFista]

[img]
QR TireSizeCalc Android [/img]

Now you can scan it instead