Breaking the Sound Barrier
This cheat lets you get up to very high speeds and "breaking the sound barrier". Here are the steps:
Concept Car
In order to get the Dodge Concept Car, which you cant buy, you have to win the British vs US race.
Faster Acceleration
To accelerate faster, rev your engine during the countdown. Be sure to go from the lowest point in neutral to the hightest point in neutral, and back down to the lowest point in neutral as many times as you can before you go.
Get some speed
Then smash a wall at a 90 degree angle
Hug the wall for about half a lap
As soon as you do that get off the wall quickly and start driving on the road. Enjoy your speed!
Hi-Fi Mode, Bonus Cars and Bonus Tracks
To get all the bonus items in Arcade mode, which include the four other tracks (Autumn Ring, Deep Forest, SSR5, and Grand Valley Speedway), all the other cars (Toyota, Subaru, Dodge, and TVR), the ending movie, and Hi-res GT Mode, you have to beat every single track with each type of car (A, B, and C) under all three modes of difficulty (Easy, Normal, Difficult.)
Super Car
Super car get a gto twin turbo (mitsubishi) sup it up all the way and it should be able to hit 248 mph.
Unlimited Money and all Cars
Enter the Gran Turismo mode. Go into your garage and get into a Dodge Viper R/10 in the blue colour. (you can get this car at Chrisler) go and start the Sunday Cup you will need a B-Licence for this. When you are Racing do the first lap without coming of the Track as soon as you did the first lap turn around and drive the other direction.
Do that one lap without comming of the track once. After you done that pause the game and press L1+R1+cross+up+down and then quit the race. Go to the garage and change your car to anything you like.
If you done this right you should hear a ringing sound under neath your screen it should say "unlimited credits". You will have unlimited credits and all the cars in the gran turismo mode.