Whats the largest size for rear tire,I love how my rear is metric and my front standard.Front is 2.75 and looks like it can take a 3.00.Wonder if rear will take a 4.00 before rubbing?

Buck

2003 xt 225 ,K&N,exhaust mod,jetted..

to the forum Buck

I've got a 3.00 on the front of my XT & a 5.10 in the rear.

Generally speaking, you can usually get away with up-sizing the tire widths by 1 size without any clearance issues.