Tire Size - does 4x10" fit a PX?

Hi all, my first post and I hope you can help me.

After putting a Pinasco 177 alu + a SIP Road on my PX125 it runs well. Plenty of torque but revving too much on full throttle. An easy way of gearing up would be to put on a 4x10" rear tyre.


So the question is really: will a bigger tyre fit or will it touch/scratch against the chassi on rough roads and bumps?

Oscar, your thorough answer has convinced me.  I´ll go for a 22 teeth primary intead of a taller rear tyre!

In fact, wouldnt it be wiser to replace the complete clutch instead of opening the old one to just change the cluth cog? The Bajaj (SIP article 93005000) or Newfren (SIP article 93002000) clutches are not much more expensive than any of the clutch cogs. Or is the quality a lot poorer?

BTW, which one of all those 22 cogs will fit  my scooter since none of the ones I´ve looked at explicitly claims to fit PX125 E. For example the (SIP article 17473300) says "not in series". What does that mean? Will it fit my clutch/gearbox?

Thanks a bunch!

OK. I probably won´t split the engine to work on intake and ports then. Since I haven´t bought the stuff yet, due to lack of funding, i´ve beginning to wonder: Why i a 7-spring clutch better than a 6-spring? Stock is 6, right? Doesn´t the 7-sping cluch has wider plates or will it fit a PX 125?

You are welcome! I made a lot of mistakes in my post because I wrote it in a hurry but I am glad to see that the message passed through anyway. Get the bajaj clutch, it fits perfectly and is better because it is a 7 springs clutch like the T5/px200. 22 cogs is perfect, the right compromise between torque and top speed. If you are going to do the job by yourself you also need these:

- 87260000

- 93320000

- 87250000

- 92130000

- 90991000

Remind that if you unscrew the brake drum nut with your vespa standing up, all the gear oil will fall out, and that the old locking washer crown nut is blocked not only by one or more external teeth but also by the internal one.


a friend of mine says yes, but it also depend on the rear tyre model (though the size should be standard, in practise there is a little variation for example between a michelin s1 and an s83) and, anyway, the difference between a 3.50-10 and a 4.00-10 is not so high in gearing up. Use the 22cogs primary gear!

Yes you have the 20 cogs, you have not to split the engine, just to open the clutch case and split the clutch. If you want only 5km/h extra you can put the 21 teeth. I know what you mean, changing tyre for adjusting the gear ratio seems much easier and for this reason I have tried a lot of different sizes (3.50-10, 100/90-10, 100/80-10, 90/90-10...) and models (michelin s1, sm100, s83, pirelli sl26 and various chinese tyres) and I waisted money and time.

But unfortunately:

a) you are not sure about the need of modifications to fit in the tyre, because as I already said, for each size there are differences between a model and another, and not always a size that should be higher or larger results to be so (for example: a 100/90-10 should be higher than a 3.50-10, but a pirelli sc30 3.50-10 is higher than a michelin sm100 100/90-10).

b) a different size than that reported on your vehicle's documents is not legal and it is very easy for the police to find it out (while it is not for a si 24 carb. or a 177cc with a standard, or standard-like exhaust).

c) if you find the right tyre which gives you the wanted gear ratio and does not require modifications to fit in, then you are obliged to mount always that size-model, without the possibility to use a different tyre for summer or winter for example.

It is much better to adjust the gear ratio with the right practice. The first time it may appear complicated to open the clutch but it is an operation that each one who have a tuned vespa should be able to do, very useful for future necessities.

Ok, thanks.

According to scooterhelp´s gearing calculator


a 4x10 tire should add approx 5 km/h at 7000 rpm to 104 km/h with the stock 20/20 carb. I really dont need to go faster than that and I don´t want to lose too much torque either.

The thought of changing primary gear to 22 cogs (20 is standard for a 1984 PX 125E, right?) has crossed my mind, but doesn´t it mean I would have to split the engine? Simply changing the rear tire just seems sooooo much easier.  Has anyone tried a 4x10 rear tire? Did it fit without modifications? How did it affect handling?

Bring it on!

Thanks Oscar, I really appreciate the initiated help! I'll buy the stuff ASAP and get on with the clutch changing. Originally I was thinking of splitting the engine so that I could open up the inlet and transfer ports as well as cutting the crank a little. But the Pinasco actually feels powerful enough as it is, even with the 20/20 carb and all... Riding faster than a 110 km/h on a PX is a little too shaky for me. When the "Need for Speed" grabs me I rather ride my old Honda CB650.

Well in my opinion there's no need to open your engine if it hasn't worked for many kms: even if with the 22cogs you'll realize that you preferred the torque with the 20 but you don't want to renounce to the top speed, there's a lot you can do. A si26 carb with T5 air filter and some tricks about the cylinder: as a matter of fact the pinasco is very quiet (too much for me, to be honest); with mmw head (with two parmakit gaskets) and exhaust transfer drilled up of just 3mm it changes character remaining absolutely reliable. Also the piston may be lightened but I suppose it would be too much for you.