Instead of using nitrous by holding it down and wasting it all at once, just keep tapping NOS as fast as you can. You will gain 1 to 3 mph with every tap, which is actually increases your speed faster than if you were to hold it down. Also, you will run out of it about one third as fast. Try this under the hardest difficulty setting.

In section 3 when you go to a drag race, take off the stability control. Note: You will be going so fast that you can get totaled.