Just in case you are not sure, the pinion is the smaller gear attached directly to your motor, and the spur is the larger gear which the pinion meshes with. By selecting your car from the drop down list, the calculation takes into account the gearbox internal drive ratio, which tends to be unique for each car make. If your car is not listed, click here to e-mail and tell us which car you drive and we will add your internal ratio.

You can use the calculator for any of the gear pitches - which are typically either 48dp or 64dp. This number simply defines how many teeth there are on the gears for a fixed circumference. 48dp has fewer teeth per centimetre, and tends to be a bit stronger as a result.

So what does changing the Gear Ratio do?

The Overall ratio you get out the calculation above, tells you how many times your motor is going to have to turn in order to make the wheels turn by one complete revolution. In other words, if your ratio was 1.0, then for every rotation of the armature in the motor, your wheels would go around once.

Assuming you've got the basics of your car set-up right, then getting the gear ratio right for your particular motor is probably the most important aspect of your set-up, second only to tyre choice, when it comes to getting your car quickly around the track. This is even more true in Stock motor racing, as it can be the difference between first and second place!

If you get the ratio wrong, it can not only make your car slower, it can also damage the motor, by causing it to overheat. Most damage to the motor and brushes is caused by "over gearing", in other words, setting the overall gear ratio to too small a number. This is because at lower ratio's, the motor is trying to get the wheels moving to a high speed as soon as you push the stick to full throttle and this puts a lot of resistance on the motor and it heats up quickly - it has been known for motors to get so hot that parts melt, or magnets become unglued from the can.

Of course it's hard to know exactly what ratio is right for each motor, but if you ask around and see how others have geared their car, you can use the calculator to see what overall ratio they are using and then choose the gears for your car to give a similar ratio as a good starting point.