Instructions
Wait for a moment in the race when you need a sudden burst of speed. Nitrous will only work in short bursts of a few seconds at a time.
Press the R1 button on your PS2 controller. You will get a nitrous injection and your car's speed will increase. The gauge showing how much you have left in your tank is located on the right side of the dashboard.
Drive into a pit stop to refill your nitrous tank. If you go an extended period of time without using it, the system will also gradually refill on its own.