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Calculating Piaggio gearup ratios
Most people think of gear-up kits in terms of the % change in the gearing they offer. This seems logical but is not quite as simple as it first seems because the exact ratio is complicated because whilst all gear kits will fit all Piaggio/ Gilera 50cc 2-stroke (except DNA) mopeds like Piaggio Zip for example have different standard ratios to other Piaggio meaning that a specific gearup kit fitted to a Zip can have a completely different % change when fitted instead to say a Gilera Stalker. The confusion can also be further compounded if you use two gear kits at once (primary and secondary)
This blog is intended to provide an easy method to calculate what the ratio change will be for different gear up kits and applies to all Piaggo/ Gilera 50cc auto mopeds with the exception of Gilera DNA. Gilera DNA uses a different style gearbox to the other Piaggio/ Gilera scooters and is therefore excluded from this blog.
This blog does not recommend what ratios may or may not be suitable for different bike setups. Dont forget an increase in gearing will only represent the same increase in speed if your scooter has enough power to pull it!!
Brief gearbox description
Because a scooter variator system is designed to operate within certain size and rpm constraints in order for it to fit and work practically and efficiently on a modern auto scooter the it's necessary for the output from it to be geared down considerably before it can be connected to the rear wheel.
This is done using four gears. The first gear is part of the clutch shaft....
and of course turns at the same speed as the torque drive assembly.
This connects to a second larger gear which is on the same shaft as a smaller third gear which then subsequently drives the final fourth gear which is part of the final drive shaft... 
and of course turns at exactly the same rate as the rear wheel.
Calculating the gear ratios
On a standard Piaggio engine the primary clutch shaft needs to turn approximately thirteen times in order for the rear wheel to turn once.
To calculate this accurately we need to know exactly how many teeth are on each cog.
There are some slight variations but generally all standard Piaggio/ Gilera 50cc mopeds (except Zip) using the following gears...
All Piaggio/ Gilera 50cc 2-stroke (not Zip, not DNA)- STANDARD
Piaggio Zip 2-stroke- STANDARD
To calculate the overall ratio we can apply the following formula...
(gear1 x gear3) / (gear2 x gear4)
so if you had a Piaggio NRG Power you would use...
(13 x 13) / (46 x 48) = 0.0765 (always calculate to 4 decimal places)
This means that every time the primary clutch shaft turns once the rear wheel will turn 0.0765 times.
Note that on a Piaggio Zip the rear wheel will turn (14 x 15) \ (46 x 48) = 0.0951 times every time the clutch shaft turns one complete rotation, this is to compensate because it has a smaller wheel than our NRG.
Now we need to compare this to a gearup kit to see what difference it would make.
We'll use this Stage6 primary gear kit as an example...
click to view...
If you buy a 'primary' gear kit this simply means that the kit consists of the first two gears (gear 1 and gear 2) whereas a 'secondary' gear kit means the kit consists of the last two gears (gear 3 and gear 4), it doesn't make any difference which you use, using one or the other (or even both) can simply offer you a wider range of gearing choices.
In our example the Stage6 gear kit it a primary kit so we are changing gear 1 and gear 2.
The listing tells us the kit is 13/39 teeth so this means that if we fitted it our NRG Power our formula (gear1 x gear3) / (gear2 x gear4) would now give us (13 x 13) / (39 x 48) = 0.0902
This means that now our clutch shaft would turn the rear wheel 0.0902 times per revolution compared to 0.0765 times with the standard gearing. Therefore the wheel would turn faster at the same revs with the Stage6 gear kit fitted.
To calculate the % change apply the formula... (new turns - standard turns) / (standard turns x 0.01)which in our example would be (0.0902 - 0.0765) / (0.0765 x 0.01) = 17.9%
Interestingly if we fitted the same gear kit to a Piaggio Zip we would get a far smaller change because Zip uses different standard gears.
We've already shown the standard Zip gearing gives us 0.0951 rear wheel turns per clutch revolution, with the same Stage6 kit fitted this then becomes (13 x 15) / (39 x 48) = 0.1042 turns which represents a % change of (0.1042 - 0.0951) / (0.0951 x 0.01) = just 9.6% !! And the difference is even greater on a secondary kit.
For this reason both primary and secondary gear kits can be fitted at the same time on mid-race Zips much more sucessfully than on other Piaggio engined mopeds although they have to be carefully chosen using the above given formulae.
We'll add a calculator to this blog shortly to allow people to enter different before and after tooth combinations for different scooters to enable people to 'try out' different gear-kits/ combinations to see how the gearing would change compared to current or standard gearing.
