Jack has put together this nice little packet about changing things like tire size:
http://dygytalworld.ehost-services13...ead.php?t=1101
Its pretty easy once you get the hang of it. The next thing is finding a tire size calculator then you got it whipped!
Pegasus has a tire size calculator. I think?
I would probably put the size tire in the tunes that you would run the most.
Lars