When starting a race on any track (This includes special tracks) with any truck you can do a turbo start without using nitros. When before the announcer says "Set" press the gass. This should give you a long nitro start without using any. This is a tricky code so you'll have to practice to perfect it.

Monster Trucks
By: Duros Bith

To get monster trucks, press the respective button once at the car select screen. Press again to show the default truck.

Punisher: Down-C
4x4 Monster: Up-C
Thunderbolt: Left-C
The Crusher: Right-C

Secret Tracks
By: Bryce Nordli via IGN64.com

El Cajon Track - To access El Cajon hold Up on the controller and press the L button and the R button at the same time. You should hear an air wrench if it worked. To play the El Cajon track select El Paso and hold the Z button and press the gas button.

Flagstaff Track - When you're at the track selection screen hold Left on the controller and press the L button to access the hidden Flagstaff track. You should hear a sound of an air wrench if it worked. To actually play the Flagstaff track select Mojave and hold the Z button and press the gas button.

Guadalupe Track - To access Guadalupe hold Down on the controller and press the R button. To play the Guadalupe track select Vegas and Hold the Z button and press the gas button.