Breaking the Sound Barrier
This cheat lets you get up to very high speeds a nd "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.

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.

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!