Good rule of the thumb for the approximate overall reduction from engine crankshaft to rear wheel (26") is the following : Divide 1000 by engine displacement (cc) .For instance a 50cc engine we get 1000/50=20 reduction.To figure out road speed vs engine rpm,
S(peed)= 0.075xrpm/R.For instance, rpm 6000 rev/min ,
R(reduction)= 20, we get 0.075x 6000/20= 22.5 mph.These figures are conservative, on the flats you might get away with using 60 instead of 50 in the formula for a bit lower reduction .For a 50 cc engine, R would then become about 17.