Calculating your Vespa rear gearbox ratio
Calculating the gear ratio of the rear end gearbox on your Vespa is a pretty easy task. I have posted some pictures here to help you understand how you can do this. all you need is a Sharpie marker. Knowing your gear ratio is good to know when your trying to build a faster top end bike for distance riding, verses one that will get off the line rapidly for intercity blasting.
First I show a bare gearbox that I have just rebuilt.
Start by positioning the gearbox so that you can look directly down at the top of the input shaft and still see the output shaft as well. Next turn the input shaft so that the keyway is pointing straight up. Now make a reference line that travels along the output shaft and right onto the gearbox housing. This will be your start and stop position.
Now begin to slowly rotate the input shaft and count "one, two, three" for ever time the keyway returns to it's upright position. As you start to count rotations you will see the output shaft alignment marks separate as the shaft begins it's one full rotation.
As you approach 10 input shaft rotations you will see the output shaft mark come back into sight. When the output shaft marks are completely realigned you stop. Then what ever number you counted off for the input shaft rotations is how many times the motor turns the input shaft to then move the wheel one full rotation. In the case of this gearbox it was 11 turns in to one turn out. Or 11:1
Below is the same method used on a gearbox that is installed on a bike. Note the starting reference marks that have been Sharpied onto the wheel and gearbox housing and also on the clutch bell and one of the variator cheeks.
Same this here as with the bare gearbox, turning the clutch bell and counting "one" everytime the bell mark aligns with the mark on the cheek.
This gearbox had 12 and one quarter input turns for one full output wheel rotations or 12:1 I have ridden both The 12:1 and the 11:1 and you cant detect any top end difference. Both are typically considered as a stock gearbox. If you happen to stumble onto a box that is in the 9's is when you will get some decent top end speed that is noticeable.
For future reference you can Sharpie the gear ratio right on the case.
I am now finished with this Wiki page. If you have any questions about the tips I suggested, or are having trouble with the process or can suggest how I can correct a mistake I have made or to make the page better, contact