Why does combustion engines need multigeared transmission while electrical engines can make due with a single gear?


So trying to figure out why electrical engine only needs a single gear while a combustion engines needs multiple gears. Cant wrap my head around it for some reason

EDIT: Thanks for all the explanation, but now another question popped up in my head. Would there ever be a point of having a manual electric car? I’ve heard rumors of Toyota registering a patent for a system which would mimic a manual transmission, but through all this conversation I assume there’s really no point?

In: 1567

Electric engines are very torquey, especially at low rpms, so you don’t need low gears to get moving. And they can rev very high while remaining very smooth and putting relatively little additional strain on the motor, so you don’t need high gears to keep the revs down on the highway.

Internal combustion engines have extremely RPM dependent efficiency. If you try to run a car engine at 10000 rpm it will give you **awful** efficiency. It will also degrade the engine faster and require more repairs.

A transmission is a way to change the RPM from the engine to a different RPM of your tires. This allows you to drive any speed from 0 to well over 100 km/h in a similar RPM range, and thus hugely improves efficiency and reduces the maintenance required on the engine. It also increases the top speed of your car, since running a combustion engine at the speeds required to go to a cars top speed would simply not be possible without destroying a conventional engine.

A transmission also allows you to trade on a constant engine RPM to lower Tire RPM but higher torque, or vice versa, depending on what you need.

Electric motors have non of those disadvantages. Their efficiency (and their torque) is almost completely independent of their rotational speed, so there’s no need for a transmission

A combustion engine only works in a fairly narrow range of rpm. They usually need at least 1000rpm to be able to generate enough power to propel a car.

The reason is that piston movement is directly proportional to rpm, and you can only fit a certain amount fuel+oxygen in each cylinder. So the amount of fuel you can burn, and the amount of power you generate is limited by rpm. There are ways to push that limit (e.g. by compressing and cramming more fuel+oxygen in), but that only goes so far. For more power, your engine needs to turn faster.

An electrical engine does not have that limit. You can supply more or less as much current as you want (until your wires start melting), regardless of whether the engine is turning or not.

So electrical engines work at lower rpm.

It also goes into the other direction though. Electrical engines have far less moving parts (no piston, valves, no mechanisms that convert piston movement to rotation, …), and thus can potentially work at higher rpm before falling apart.


Electric motors generally have good torque and excellent efficiency over a wide range of rotational speeds. Having a single set of gears or none at all generally doesn’t effect performance. In addition transmission gearboxes are heavy. Typically the added weight of having a transmission with several gears has been considered extra weight which carries a major penalty on electric cars, while not providing much benefit. By removing such weight one could add extra batteries for example, at a similar cost which boosts range.

Internal combustion engines like Diesels or Petrol engines have poor power and low torque at low speed, and they have very poor efficiency and reduced reliability at high speed. They are generally most efficient and produce the most torque at a relatively narrow range of speeds. Therefore it’s proven to give better performance by having an adjustable transmission with several different gear ratios between the engine and the wheels.