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Picture of how to supercharge a cheap radio control 2
This is a more descriptive instructible than the last one about making a faster radio control. This instructible is only for RCs that are 2 wheel drive or only have 2 motors. (If you are not entertained by it as a slow rc and don't mind the possibility that if you mess up too much then your rc could be broken then continue. If you reduce your RC's value I am not responsible.) First measure the gear box and get an approximate length of the area you have for the motor. If you can not see if you DO have about an inch and a half from the motor edge of the tire then go to this instructible, for instructions on how to mount a bigger motor; if you want a harder job that will make a faster RC if you use the right motor.(http://www.instructables.com/id/how-to-supercharge-a-cheap-radio-control/) 

Step 1: MORE POWER

Picture of MORE POWER
Then, find out the voltage the car uses also find the Mah rating if it is rechargeable. If the voltage is 6 volts and under then double the voltage in battery rating is required. If it is 6 volts and over then give it 5-6 more volts of battery rating. If the RC is alkaline battery powered you will need to make it rechargeable battery powered. If your batteries are not rated at at least 1000 Mah then double the Mah is needed of the batteries you will use in the car.