The smaller the wheel, the slower Feet per Minute speed, so a 3400 rpm with a 4 inch wheel is close to the actual speed of 1730 rpm with a 6 inch wheel. FPM is what is really important, so if you go with Harbor Freight, use 4 inch wheels. Harbor has some fairly good wheels. You want spiral sewn, and as "tight" a cotton as possible. Felt is even better, but expensive.