I have 9:00X20:00's,on my 6500.(2-speed rear). Top speed is 55.(Hi-side). I wouldn't push the engine past its "Happy Zone". I still have a 1/4" left in the pedal travel,so I could go faster,but I don't want to shell a GOOD 261,that's survived 58 years. You may use the same rims,since the diameter of the tires stay the same,just a taller sidewall/profile. This may get you up to 50 M.P.H.,depending on your rear-end ratio.
