Unlock all cars
At the options screen, press Triangle, Square(2), X, R1. If you entered the code correctly, you will hear a sound.
Hidden car
Choose any car, then hold R1 + R2 until the race begins.
Buggy
Choose any car, then hold R1 until the race begins.
Truck
Choose any car, then hold L1 until the race begins.
UFO
At the car selection screen, hold R1 + R2 + L1 + L2 and choose a car. Keep those buttons held and press X at the transmission selection screen. Then, hold Triangle until the race begins.
Change car colors
At the car selection screen, press Square.
Reversed tracks
Once circuit mode has been completed, an option for forward and reversed tracks will be selectable.
Stunt mode
Once tournament mode has been completed, you will be able to access a stunt mode option on the Golden Gate track.
Bonus cars
Complete one of the following goals to unlock the corresponding bonus car:
Muscle car
Find 4 keys in any track. Then at the transmission selection screen, press Down.
Hot rod
Find 8 more keys in any of the other tracks. Then at the transmission selection screen, press Down.
City bus
Find all 16 keys. Then at the transmission selection screen, press Down.
Police car
Complete tournament mode. Then at the transmission selection screen, press Down.
Taxi cab
Complete circut mode. Then at the transmission selection screen, press Down.
Shortcuts
On Track 1:
After going through the tunnel, go throught the fence to the left. After going through the city bear to the left shoulder. Right after the finish line go into the grass and go through the little tunnel.
On Track 2:
Go through the little tunnel to the left near the beginning. Instead of going right, go left off the ramp. Go under the truck to get the key.
On track 3:
In the beginning, go straight through the building. After the big jump, go over the little one and go left (key). After the long stretch of road keep going straight.
On track 4:
To avoid the sharp turn, after going down the ramp, go right off the ramp. Go left on the fork, and then go left through the tunnel.
Stunt Race
To enable stunt race, win circuit mode. After you have won circuit mode, go back to a regular game. Scroll to the weather settings until you see stunt race. Now you can play stunt race mode. Note that this only works with track 1.