Vespa gearing...



I have a PX 125 engine with a DR 177 kit on it.  I know the gearbox is standard PX 21/67 primary but it has a 35 tooth 4th gear and basically is struggling to keep its speed up on any inlcine.

If I want to reduce or shorten top gear to make it rev more, and without taking the engine apart, can I do this by changing the clutch primary gear?  If so do I want to go from 21 up to 22 teeth, OR from 21 DOWN to 20 teeth ?

you are welcome for the help.


there is a nice helpful gearing section and p-series gearing calculator at ( ). other good scooter info there too.

to shorten the gearing you want to go down to a 20 tooth.

what exhaust are you using? it seems like a DR177 should be able to pull and hold 21/67 even with a 35 tooth 4th gear. my stock Stella 150 2T (LML Star 2T) had 21/67  and when i put on a SIP road exhaust i also up geared with a 22 tooth and i have no problems keeping speed.

Thanks for this.  I had guessed 20 tooth, but was a little confused working things out. The engine has a sito plus exhaust on it.  I am not  a lightweight either.  It runs OK most of the time, but up long slow hills it loses speed.  I just wanted to try down gearing it a little to see if the shorter gears helped it hold speed a little better.


many thanks for the help