When I was building my variable speed buffer the man who was of so much help provided me with this information. I hope it helps you; I've put the important part in bold;
Roy, take this little formula to figure your speed:
rpm of motor divided by the diameter of the pulley.
Use this as an example.
1750 rpm divided by 6" pulley=291
291rpm divided by 3" pulley = 97 rpm
A 10 inch wheel will run 97 rpm. So will a 6 inch wheel.
The surface speed is a different matter though. You might want to drop down to a smaller wheel:
A 10" wheel at 97 rpm is running 3,048 inches per minute.
A 6" wheel at 97 rpm is running 1,828 inches per minute
Formula to figure surface speeds of wheels:
10" x 3.1416 x 97 = surface inches per minute.
6" x 3.1416 x 97 = surface inches per minute.
I ended up using 4" wheels and love them! :y