If you want to make your RC car go quicker, an easy option could be to just increase the voltage going to the motor through the ESC.
More volts means higher speed. This in theory is correct but there may be a few things to consider first.
Most RC cars can run on a range of different voltages. But there will be a maximum safe voltage that your ESC can operate at before it will just kill it.
Also the motor will have a maximum voltage that its rated to.
The ESC may be rated up to a certain voltage but the motor may be able to handle a higher voltage.
The top speed of the car can then be improved just by replacing the ESC with a high voltage one.