An EV has no transmission

The AC motor's rated speed, given in revolutions per minute (rpm) is directly proportional to the frequency of the current supplied (in the motor's operating voltage and current range). In case of a motor connected to a standard U.S domestic supply this is a fixed 60 Hertz. But in an EV, the rotational speed can be varied by independently varying the frequency of the current and the voltage supplied to the drive motor based on the torque and speed required at that moment.

The reason an EV does not require a transmission for most situations is because whatever the rpm of the motor, it produces reasonably good torque across the range and losses due to heat and friction are low. But in a gasoline engine, we need proper gearing that is appropriate for the rpm to compensate for the lack of torque at low RPMs. Without gearing, it would be difficult to maintain speed or torque in a usable range in a gas vehicle. Heat and other losses are also very high in an internal combustion engine -it is only around 30% efficient in converting fuel to output.

Some EV electric motors can operate at very high rpms, even up to 18,000 rpm for short bursts. However, at these high revs proper cooling of the rotor and bearings become important. An ICE car on the other hand typically has a limit of only around 5,500-6,500 rpm. This limit is called the redline of the engine. 

All is required is a reduction gear as a final drive to reduce the motor's output to the wheel. The conceptual figure below depicts an example of a reduction gear how the rotational speed is reduced at the wheel by the larger gear. The ratio of the number of teeth of the larger gear wheel to the number of teeth in the smaller gear wheel gives you the gear ratio of the gear train below.

In this example the final drive ratio is:

                                                                           24/16 = 1.5:1


The final drive ratio in EVs are typically higher.