Anyone who has driven an Internal Combustion (IC) engine is familiar with gearbox even if you drive an automatic one. Beyond providing for a change in the drive direction, the reason for a gearbox is simple. By its design, an IC engine can only reliably spin up to a certain mark, around 7,000rpm for the average petrol-powered road car.
They also only make usable and peak torque in a narrow rev band. Thus, a gearbox is used to vary the effective torque and speed as required by differing driving conditions. For instance, at start-off, a large gear wheel is used, which provides more leverage and thus more torque that’s needed to overcome inertia. Conversely, at high speeds, a smaller gear wheel is deployed which spins faster, thus providing more vehicle speed. In this way, different gear sizes provide differing amounts of torque and speed while keeping the engine spinning within the meat of its power band.
But for an electric motor, it develops its maximum torque from standstill and stays pretty much consistent throughout its revolution range. Furthermore, this rev range is very broad, easily and safely spinning to as high as 20,000rpm. Thus, with such a wide window of operation, one gear ratio is sufficient to provide the torque and speed required.
This does not mean, however, that a multi-ratio setup isn’t possible for an EV. With multiple gears, an EV could achieve better low-end acceleration as well as top speed, and this could also help increase its range marginally.
In fact, some Formula E race cars have used gearboxes with up to five speeds, while the Porsche Taycan employs a two-speed gearbox.
However, given that a single ratio provides the bulk of the required performance, nearly every EV today uses a single-speed unit rather than have to manage the complexity of a multi-ratio gearbox as well as the added cost – the last thing an EV needs. And this is why EVs today have no gearbox, or rather, just a single speed unit.