It wont help with recalibrating your speedo but I find this usefull for calculating tire size versus rim size on overall diameter and how much it will affect your speedo

http://www.miata.net/garage/tirecalc.html

cheers
barri

__________________
61 Austin mini
67 Lotus 7
74 450sl
76 Cadillac 8.2l (501 ci)

some new cars

megasquirt conversion on:
djet 74 450sl http://www.mercdjetmegasquirt.britautorepair.com/
cis 76 450sl http://www.merccismegasquirt.britautorepair.com/

the best view is always from the point of no return