I have a 120/90 on mine currently, its a fits, but you certainly wouldnt want to go any larger. Haven't run it through heavy clay yet to see if she clogs up, but so far so good. It was on an old bike up at my farm which i dont use often, and was 90% on the tread, so i thought id run it on the TTR. 120/80 might be better or 110/100.