here are some numbers i crunched to come up with some general top speeds for different sized roller. of course this asumes you engine has the umph to turn at 6000rpm while loaded and also factor in some loss of energy to friction and wind. this should help some folks out. tire size doesnt matter much with friction drive, except a taller tire traverses variances(pot holes, curbs, debris) in the road much easier than a small say 12"-16" wheel. but thats not to say small wheels wont work just that there not ideal.
mph at 6000rpm
roller dia. Speed
1" 17mph
1 1/4" 22mph
1 1/2" 26mph
1 5/8" 29mph
1 3/4" 31mph
mph at 6000rpm
roller dia. Speed
1" 17mph
1 1/4" 22mph
1 1/2" 26mph
1 5/8" 29mph
1 3/4" 31mph