It is really important that a proper transmission design is completed, to get the required performance and top speed. Knowing the maximum rotational speed of your chosen motor, you can choose the top speed of your EV by selecting the right size wheels, differential (if one is being used) and the fixed ratio reduction gearbox.
This spreadsheet (Microsoft Excel) allows you to adjust various parameters: TransmissionDesign.xlsx
Reducing the top speed with a lower gearing will improve the acceleration. Another way to improve acceleration is to reduce the mass of the vehicle. A careful balance is required between top speed and acceleration though. You will most likely use the acceleration a lot of the time but, how often do you drive your car over 80mph?
In an EV conversion it is possible to retain the manual gearbox, to enable more flexibility and performance.
Rob did a detailed transmission design for his Fisher Fury R1.
In the MX-5e we are using this fixed ratio reduction gearbox on the output shaft of our Nissan Leaf motor. It was manufactured by Inductive Autoworks in the USA and has a reduction ratio of 1.8 : 1. It has an output flange that matches the differential input flange with four M10 bolts on a 96mm PCD.
This is filled (600ml) with fully synthetic automatic transmission fluid (ATF). We are using Mobil ATF Multi-Vehicle Premium Fully Synthetic Automatic Transmission Fluid in the MX-5e. It also needs an oil breather pipe.
The propshaft as fitted to the Mazda MX-5 1.6 NA is 106cm long, with a tube diameter 57mm. Both ends of the prop fit a flange with four M10 bolts on a 96mm PCD. We weighed the old prop at 5.6kg.
With so much torque available, it makes a lot of sense to use a limited slip differential.
With the MX-5e we have upgraded the differential. As a 1992 Eunos Roadster, it came with the earlier and smaller differential with a 6" crown wheel. These are known to be a weak point in modified MX-5s with higher power, so we are fitting a later (2003 in our case) and larger differential with a 7" crown wheel.
Another big reason to change the diff is to get a better final drive ratio. Our new differential has a final drive ratio of 3.636 : 1, giving a better top speed.
We are using Mobil Mobilube 1 SHC 75W-90 Fully Synthetic High Performance Manual Gearbox Oil in the MX-5e differential.
It also possible to upgrade a Tesla SDU (but expensive).