Just think of it as riding a 10 speed bike. When you're on the big front sprocket, and a smaller rear sprocket, your legs (the engine) are making alot more revolutions for each that the rear sprocket (axle), giving you "acceleration", thats why going up a hill is easier.
Its the same principle.
Your legs and the engine can only make so many RPM. Thats why when you shift your bike to the smaller front sprocket and larger rear sprocket, you can go faster, as you can turn more RPM having a smaller sprocket in the front, and the larger rear sprocket.
You need to find the gear ratio that matches what you want the car/ bike to do. Take off, or go fast. It depends on the track you are running what the best ratio is.
