Press B + R when in drive in original, arcade, or crazy box modes for an extra boost at any time. Note: Press R + [Plus] when using a race controller.

Use the following trick to get even more speed than a Crazy Dash. You can do this at any time, but only if you are riding and did a Crazy Dash. When riding, release R to stop accelerating. Then put your taxi in reverse, and immediately after that press B + R to do a Crazy Dash. This should result in extra speed. You can do this as many times as needed for even more speed.