Gran Turismo Cheats (PlayStation)

Go backwards on high speed ring for three laps if you don't go off the track or hit any
cars when you pass the finish line the third time you should here a car
  horn then at the main menu, you will have 1000000 dollars and the concept car.

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.

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

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!

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 get a gto twin turbo (mitsubishi) sup it up all the way and it should be able to
hit 248 mph.

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.

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.

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!

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