Depends on your weight and passenger if any but I'd start at 24F, 24R and go up from there if you're 160-200lbs say. The manual says 22/24 for original K81/TT100 4.10x19. That's too low for AM18, 20/21 but might be OK for your fitments. The 3.25 front would need a couple of pounds more than a 4.10 anyway.