Vespa P-Series Gearing Calculator
The scope of Vespa gearing is less broad than Lambrettas but still complicated. There are 2 things that can be changed to result in higher or lower than standard gearing. The primary drive consists of a smaller helical gear that is integral with the clutch assembly and a larger helical gear riveted to a cluster of 4 toothed gears.
These must be installed as a matched pair except in some cases where a free gear with one less tooth can be added to the output shaft and still mate with the cast gear cluster (such as a T5 4th in a P200 gearbox. Lastly the tire size can be adjusted with slight modification to fit a nonstandard 4.00x10 tire. I also included a 3.00x8 tire for anyone who replaces their engine with a P-Series one on an older 8" wheel large frame and wants to keep the original 8" look.
Below are calculators for the three main Vespa gearbox options that will give gearing for a range of standard Vespa gearboxes without mixing separate gears from different models. These are currently only for P-series bikes. If anyone has info on 150 Supers and other 70's bikes please email me.
I have included some primary drives kits by Malossi and Polini. P125 & 150 ratios have also been shown in the P series calculator as they can be modified to fit a P200 clutch by unriveting the central clutch gear from a P125 clutch and reattaching it to a P200 style clutch. The final ratio of a conversion like this will be pretty low but try it and see if it works for what you are trying to achieve. The resulting page will show gear ratios for each of the 4 gears and show an RPM to speed analysis at various engine revolutions. Also I have added some Vespa Cosa clutch options that I was sent by email.
P-Series Gearing Calculator
Miles per Hour
Kilometers per Hour