Start with Two Cars:
Go to the simulation mode, go to Nissan, and by a S13Q. You should have enough cr to get a S13K. Enjoy your two Nissans.

Breaking the Sound Barrier:
This cheat lets you get up to very high speeds and "breaking the sound barrier". Here are the steps:

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!

Concept Car:
In order to get the Dodge Concept Car, which you cant buy, you have to win the British vs US race.

Easy Money:
The easiest way to get lots of money later in the game is to race the qualifier for the all nighter. You get $20,000 just for qualifing and it is very easy to do this with the GTO special edition or any other car with good speed/acceleration and handling. It takes app. 1'45" to complete with the GTO special edition.

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.

Nissan Nismo 4000gt-r:
To get Nissan Nismo 400Gt-r you have to get all golden trophies in International A class license.

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.)

Hidden Video of the GT Staff:
In Arcade Mode, first get the hidden tracks, then, on every track, take 1st place with any car in A,B, and C class mode on Normal setting or higher. Then go to Bonus Items, you'll see "Staff Video" on the menu.

Free Parts:
It seems there is a bug in GT which means you can make free duplicate copies of any parts you buy for a particular type of car. Try this:

Buy two identical cars. Give them a different color scheme if you like.

Buy loads of parts for the first one - fully spec it up if you like.

Check out its list of fitted parts from the garage (you should see a list of what you just bought.)

Switch to the other car and get in it (the one which you haven't fitted any parts on yet) and check its fitted parts list (should be empty).

Still in the second car, go race (Spot Race will do).

When you get options to qualify, go to Machine Setting, then Change Parts. You'll now find you can select all the parts from the other car, tyres, turbos, suspension, intercooler etc, in effect fully spec'ing up the second car.

Exit the race, and go back to the garage. Check out the list of fitted parts for both cars. The parts are now installed for both of them!

NOTE: This won't work for the Weight Reduction or Racing Mod options and a few other things (like port polishing), as these are not available from the Change Parts menu - they're not really "parts" as such anyway. Also, it doesn't seem to work with cars that have been won.

Of course the best thing you can do with this is sell the extra cars. As it cost you nothing to increase its specs, you'll make a profit when you sell it!

Easy Start:
When you first start and have 10,000$ to burn,go out and buy a Honda Prelude '94 VTEC cause that car has serious handles.Get your B class liscense.Win a couple of races to get around 5,000$ to buy Flywheel and Muffler Upgrades.Race the Sunday Cup and win ALL the races at 1st place.By the time you win the Championship,you should have about 45,000! Keep repeating if wanted.I was able to get the Dodge Viper after 2 Championships!

High-Powered Cars:
Are you tired of slow,but expensive cars like the NSX and the Aston-Martins? Well,go to Mitsubishi or Nissan and purchase either a GTO Twin-Turbo from Mitsubishi or a R33 Skyline from Nissan and have tons of cash.Both of these cars can be upgraded immensely and their horsepower go above 900! These cars also have superior handling.Want handling and speed? Buy these two and you will never lose!

Hints:
The Chevrolet Corvette Grand Sport in the game is phat. If you get $80,000 you can turn it into a racing modifaction. When you do that you can change the color of the car. The same look will go on the Chevrolet Corvette Coupe. I recomend you use the Grand Sport.

The Fastest Car On The Game:
You want supreme dominance? Buy a new Supra turbo and tweak it to the max. you now have a 910hp RWD car that can really move if you put some downforce on it. This car can essentially go undefeated on the Megaspeed race. Note, soft tires,zero camber,loads of downforce and super large 1st and 2nd gears are neccessary if you dont want to look at the turn you just came out of in front of you.

Specs? how about this:
0-60 in about 3.5 sec
top speed 267 with proper gearing
can go around the long turn at High Speed ring steadily at 210+mph

How to get the R35 GTR special quad turbo skyline:
To get this 1290hp car it is simple, you have to race rome short course, come last three times in a row, 2nd two times a row and 1 st once, after you win the screen will will go back to the main menu and a car will be spinning around slowly, and just press x to get into the car. This car is so fast and yet so light at 978kg. Enjoy your car or evan sell it only for $100. PS. You have to do this with a STI 22b model.

Better Brakes:
To get better brakes is kinda tricky but it gets simple after a while. All you got to do is press triangle instead of square. With triangle you only have to press it for a short second which will slow you down just enough to complete a full turn. Unlike square which will make you come to a stop very quickly which is usually not needed.