I'm trying to figure out gearing on the YX140 I'm putting in my CT70. From what I can find, the transmission ratios are 1st 3.272, 2nd 1.937, 3rd 1.35, and 4th 1.04. and a primary reduction of 18/67 or .269, and makes max power at 7500 rpm. But when I plug these numbers in, it doesn't seem right. I'm running 120/90-10 tires, which have a dia of 18 inches. I run a 31 tooth rear sprocket and have 13 thru 17 front sprockets to choose from. But even using the 17 tooth up front, at 7500 it works out to only 57 mph. I thought it would be a lot faster. I checked my calculations twice, so I'm thinking my specs must be wrong somewhere, possible the primary reduction. My goal is to gear it for top speed at redline in 3rd and use 4th as a crusing overdrive. I thought I could eliminate all the trial and error. The 110cc it's replacing was using a 14 tooth front sprocket and that gearing was too tall for it to pull top speed in 4th.