Did I get this right? if not what is the way please write it as if using a calculator. I don't understand math. 52 sprockets / 12 sprockets on the rear= 4.333 ratio? X 60rpm crank speed x (20â€ x pi=62.8inch circumference) = 16327.987 inch min. / 12 inch = 1360.6655ft/min. and then 5280/1360.6655 = only 3.88 mph???