Originally Posted by 1slowride

Sure there are other ways to figure **** but this is how I did it. Took tire diameter x Pi to get the tires circumfence 7.22'. Then figured out how many ft per min you travel at 1 mph which came out to 88'. Now figure your front / rear tooth ratio IE 39 front divided by 25 rear gives you a ratio of 1.88 (rear wheel turns 1.88 times for each turn of the crank. Now pick a speed you ride say 8 mph. 8 mph X 88 ft traveled per mph= 704 ft traveled in one min. We know the tire circumfence is 7.22'.
Divide the 704 ft travel per min by 7.22' = 97.5 turns of the rear wheel per min. We are almost there, Take the turns per min of the wheel and divide that by the gear ration. 97.5/ 1.88 which give you a cadence of 52 rpm. I put this into an excel sheet so I can just plug in the different numbers and it spits out the info. Be happy to fix this sheet up a bit and send it to anyone who is interested. There is one thing I did not check. That is the true diameter of the tire, I just used 700mm, was too lazy to get out of bed last night to measure it, shouldnt make much difference. Any way it still works as it shows you the relative diffence between different ratios.