The standard ET4 scooter is lower geared than the performance of the engine will cope with. Most automatics are biased for round town usage with minimal wear on the clutch and transmission from continual stop/starting. This reduces the top speed the engine is capable of achieving.

A lot of ET4s spend time on bypasses and A-roads running at high speed. In order to improve the top speed and the ability to cruise at higher speeds more easily there are several gear kits that keep the engine operating on the transmission pulleys to a higher speed. This maintains the power output at these higher speeds so that top speed is
improved.

Depending on the conditions the scooter is used under, there is a choice of gear kits to suit the usage:-

If the scooter is only used around town and spends all of its time under 60mph then there's no need to change the gearing.

If the scooter is used in general mixed conditions, spending a time above 60mph and a lot of use in traffic then the 10% up-gearing will improve the top speed, reduce the engine revs at top speed, and will show no difference accelerating from a standing start.

If the scooter is only used flat out up a motorway and the driver is very light then the very high gear kit (+25%) can be used to maximise the top speed. This will affect the acceleration from a standing start up to perhaps 15mph. If the scooter is not used around town then there are no disadvantages.

The intermediate gear kit (+17.5%) loses a small amount of performance on initial acceleration but gives more top speed than a 10% ratio.

Obviously, what you wish to achieve from the scooter determines which is the optimum gear kit for you.