[Alan Anderson]

Do you want to limit the maximum speed, as if you had an extra stage of reduction in a gearbox? That's easily done by multiplying the joystick values by a number less than one. You lose the ability to apply full power if you do that, so it might be a good idea to include the option to leave the joystick values alone if a "turbo" button (e.g. the joystick trigger) is pressed by the driver.

Do you want to change the response of the joysticks so that they are more sensitive at low speeds, but still go to full power when pushed all the way? That's a little more difficult, but still straightforward. Try multiplying the joystick value by itself three times, yielding the cube of its value. That response curve is something many teams have used in the past.