Overdrive is generally speaking any gear that multiplies the input to deliver the output. For example in your road car, say gears one to four would transfer power to the wheels such that the wheels spin slower than the engine. Top gear (or fifth) would then be an overdrive which would transfer the power such that the wheels spin faster than the engine. An overdriven wheel then has power transmitted mostly as speed rather than torque. In this light then your question needs to be qualified in relation to what you're trying to achieve: speed or torque? If you want a tractor puller then overdrive is out of the question. If you want a speed monster then you need as much revs to the wheel as possible. Whether the overdrive ratio needs to be 2 or 5 or whatever depends on many other variables (motor power, wheel size, car weight, etc.).
In conclusion, you need to tell us more or research more and refine your question before an answer can be hammered out.
Team Greasy Weasel
The best upgrade to any car is some driver skill.