Final ratio.
The overall ratio from drive spline to transmission to output spline. The transmission itself gives you 12:1, PLUS the reduction in RPM applied from the output sprocket to the drive wheel/axle. Example.
7000 RPM @ 12:1
12:1 reduces the RPM out of the transmission to 583.3~ RPM. An 18" tire is roughly 56.5" in circumference, so at 583 RPM, you'll travel 32939.5 inches per minute, 2744 feet per minute, 45.75 feet per second, which is equal to 33.58 MPH at max RPM, 1st gear, 1:1 transmission to axle ratio.
Fast. But a problem BECAUSE it's fast in 1st. You want torque in the low gears for take-off. Having the torque graph would help a ton, but instead, you can assume 125cc at 12:1 final will be an absolute dog off the line, if able at all. The trick to increase torque is to pick this up in the ratio between the output spline and the axle/drive wheel. That's why they did it on the motorcycle it was on, and that's why it has a 12/8/4 progressive 3 speed with what I'd assume is at least an 8 inch rear sprocket.
You need to hit the drawing board, find what top end you want, what's acceptable for torque, and plan for the amazing range you'll have to play with by having a 3 speed. My opinion is make it torquey as hell and let that 4:1 in 3rd stretch your legs for you. That way you're cutting ruts AND blowing people's doors off.
