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.

Using a speed calculator that I have here, the results I found are below, and based upon the numbers you provided above. The top box is a dropdown menu-style, the selections shown are just to get the numbers to appear in the second box. Those numbers can be adjusted for the different gear ratios in the trans set and primary gear reduction. The 3.722 shown below is the result of 67/18. The tire diameter was adjusted to 457.2mm (18")

You make max POWER at 7500, but you won't reach max SPEED until you reach max RPM. Provided that you have it geared correctly...so that the engine CAN pull you to max RPM.

To add to what kirrbby said, so you should gear it to pull max rpm IN 4TH! 17-31 will gear it to be a DOG in 4th. Slap a 16 on it, that will help without being too buzzy. Probably not the ultimate for top speed, but close.

Hey dirtbkr88, thanks for the great chart, and based on that my calculations, comes out pretty close to 57 mph in 4th at 7500 rpm. But I'm not confident in the primary reduction gearing ratio. I expect the bike to do over 60 at max rpm. Is that about 8000 rpm or what? I don't mind the bike being a dog in 4th, if it'll pull top speed in 3rd and it's not too much of a dog off the line. For example, let's say it tops out at 65 mph at 8000 rpm. Then cruising at 50 would be about 6150 rpm. If it would do that in 3rd, than a shift to 4th would drop it to comfortable 4700. Now I just don't know if the engine has enough torque to pull 50 mph at that low an rpm. I have every countershaft from 13 to 17 teeth so I guess it'll be trial and error.

I am very happy with 16-32. You posted you are interested in a larger counter. 18 will not fit unless you grind the case. Post up what you settle in with. Everyone likes pictures, here is a 18 on a YX.

These engines are built to run at sustained high rpm, so you gain nothing by lugging it at lower RPM to get a speed. The difference between these chinese lumps and the original honda is what you can do for maximum sustained RPMs. The 140 can hit about 10k before the valves start floating, so let's say 9500 is your max RPM. Your max SUSTAINABLE rpm...the RPM where you can hold it there forever and the thing won't come apart is somewhere between 6500 and 7000...the honda ones slightly higher to 7500 RPM. A properly geared CT70 with a 140 in it would be to target 55mph at 6700-7000 rpm depending on your weight and riding preference. and pretty sure you have the 4th gear ratio incorrect. I believe the gearing on a 140 is a 24 main shaft and 22 secondary shaft yielding a .92 ratio.

What you say makes sense, in that a target of 55 mph at 6700-7000 rpm. But since this motor supposedly makes max power at 7500, I have no desire to spin it much faster than that. My CL90 has a tired Lifan 140 in it. The rear tire is a 2.75-18 with a 40 tooth sprocket, which makes it withing 3% of my CT70 120/90-10 and its 31 tooth. My CL has a 16 tooth countershaft and has pretty long legs. It'll pull 55 mph in 4th and will pull that in 3rd with the engine screaming as well. However at 45mph, it feels comfortable in 4th, and too busy in 3rd. Since the Lifan starts with a easy kick compared to the YX, I expect the new motor to have more torque and might even tolerate taller gearing. So on the CT70, somewhere between 16 and 18 teeth should get me where I want to be. And the 18 tooth countershaft I ordered came in today and it fits and clears.

I see see you ground the case a little, does the cover clear also? I didn't try that yet. How many miles did you get out of that tired L140?

It cleared fine, but I did file a bit off the top boss, since the sprocket does wobble a bit on the spline and the chain may slap as it stretches. I got 5000 miles on the Lifan with no maintenance aside from oil changes and valve adjustments. It still runs fine, except it weeps a little oil and the shifting has gotten a little notchy.