Easy Money:
This isn't a cheat so much as an effective strategy for beginners. First get a car to the 300 hp range, then race the Super Speedway track in the Beginner class GT World tournament. If you race against only Rally cars, you should be able to win, for $10,000 a win. Do this about 10 or 15 times, until you have enough to get a good car and tune it into the 500 hp (or higher) range. From there, enter the European Championship series in the Amatuer class. Win the first four races, then save your game. Enter the fifth race and immediately exit. You'll win the series and win one of four bonus cars. If you get a car other than the Vertigo race car, exit out and load your game to quit the fifth race again. Do this until you get the Vertigo race car, which you can sell for $300,000. You also should win about $50,000 from the four races and the series win. This should take about 20-25 minutes, and yield around $350,000.

Easy money:
Buy a car such as Skyline and upgrade it to the max. Then enter the Superspeedway endurance race. Have 2 tooth picks ready. Stuck 1 into the accelerate button and another one into the direction pad. In about one hour, you will have both the prize money and prize car waiting for you.

Easy AutoPilot:
You will need two things: Scissors and tape. On any circle races, like the Super Speedway and Test Track, you can use a homemade autopilot to beat the race. It's simple; Just tape down the X button as hard as you can, then tape the D-Pad right slightly but not too much.. (You really only have to tape the D-pad on Super Speedway because if you let the car just go, it will go into the pit.) Now watch as your car gracefully goes around the track without your help. The only thing you need to do is turn off vibration so the tape doesn't come off. This autopilot works especially well on the Endurance super speedway race.

Easy money:
You can race the endurance races as many times as you want and you'll get a prize car every time you win. Easy money, plus mutiple high power cars.

Getting the car you want from any series race:
When playing the series races there is a total of 4 cars that can be won every time. In order to get the car you want you must be ahead in the series by about 20 points. When you come the screen right before the last race you save the game in the series. Start the last race and exit while in the race, NOT the staging screen. Keep doing this until you get the car you want.

Free upgrades:
This is not a cheat but a way to make the game a little bit easier. If you and a friend are both playing GT3 then split the cars you buy, one buys a FF car and one buys a FR car, then upgrade them to the max and go to the trade section in your garage area. The cars trade for the purchase price but you get all of the upgrades done to the car for free.

Free upgrades:
You can also sell the cars that you have to yourself by clicking the trade button and it is just the purchase price

Pro difficulty level:
In the select difficulty level screen, point the cursor on 'Difficulty' and push L1 + R1. Do so, and you get the Pro difficulty level.

Make the more difficult non limited races a breeze:
Save money and as soon as you are able to purchase a powerful turbo car like the SKYLINE do so. Then buy upgrades as you can. Once fully upgraded race the Superspeedway endurance race as many times as it takes for you to win the Formula One race car. It's acceleration and handling are bested only by a couple other F1 cars which you can now win with ease.

Raw power:
If you really want to see raw power in action do what I did, Buy a Mazda Mx5, do it up enough to win a Lancer 6 Tommi Makkinen Edition, win all the cups available, win the 130miles endurance race, get a XR8 and 200,000 dollars, buy a Lotus V8-S3 Turbo, chuck a stage 4 turbo, 6-speed gearbox, race the 300 laps enduro and watch as you can lap 2nd place 11 times. Top speed of 422K/s

Easy money:
Get a rubber band. Go to options. Set the accelerator on the right analog on the leftside. Take the rubber band and wrap it around both analog sticks. Make sure you have the Pikes Peak edition car or another super machine. Go to the Superspeedway endurance race. After you apply the options and rubberband you should hug the outside of the track and the gas will be maxed. Then you should have no problem winning the race and win 200k and 1 of 4 killer machines.

Cheap HP upgrade:
When you get a new car, before racing it take it to the GT shop for an oil change. If the car has over 400 hp you will usually see about an extra 30 horse.

Full Replay Control:
During a replay, press start. At the menu, select the music-enhanced replay mode. (the one with the music note on it...) then press start to continue the replay. Now, open the PS2 DVD drive (eject disc). Now you can control the replay camera engles and effects yourself. Just press any buttons, see the result...its cool. You can also do button combinations to create some cool replay effects. Experiment and see what you come up with (when the replay is over, be sure to close the PS2 DVD drive door, or the game won't be able to load.)

Max Speed:
Get a car with 1000+ horsepower. This cheat works in the Escudo, C5R. The Viper was close, dunno about other cars. I hear the GT40 will do it. On the test track, set up the car with the lowest possible downforce in the front, and the highest in the back. Gear it way up, put the final drive ratio down near 3.0. Set the spring rate at its softest level and drop the ride height the whole way. ASM and TCS should be off.

Jump on the test track for a max speed test. Go backwards for a few laps if you want to test without restarting. Once you get to about 235 mph, the front of the car will lift off the ground. Once it lifts the whole way, the car will begin to accelerate through 300 MPH, 400, 500 plus. Only stops when you hit the wall at the end. In the Escudo, I was able to hit 500 mph consistently in the speed tests. During some 15-20 lap races, I came off the corner just right and got up above 800 mph. My max speed test is at 526.6 mph.

Good car:
Someone is talking about the lotus v8 with lots of horsepower. It is fast, really, but very hard to handle. An very good alternative is the Altezza LM race car. This car you can win in the Altezza championship in the Amateur League. With this car, you have 1002 hp, and at the same time adjustable down force and so, which gives you al good handling with great power.

Suzuki Escudo Pikes Peak Version:
Save much money as possible to get yourself Suzuki Escudo Pikes Peak Version and upgrade it to max. This will cost you 1,014,000 CR (including car). Use this to drive in the rally and you'll breeze through the rally course. If you get gold in all races which you should do with this car you'll get another Suzuki Escudo Pikes Peak Version as bonus.

F687/s:
Successfully complete the 100 mile Seattle Circuit Endurance Race

Zoom:
At the car select screen you can hold the R1 button to zoom in on the available car. If you hold R1 + R2 you will zoom in even further (this will demonstrate the amazing ability of the PS2 to "push" graphics).

Unlock cars:

Aston Martin Vanquish:
Win the British GT Car Cup in the Professional League.

Audi S4:
Win the Tourist Trophy in the Amateur League.

Audi TT 1.8T Quattro:
Win the Tourist Trophy Audi TT Race in the Beginner League.

Celica TRD Sports M:
Win the FF Challenge in the Amateur League.

Chevrolet Camaro SS:
Win the Stars and Stripes Grand Championship in the Beginner League.

Citroen Xsara Rally Car:
Win the Super Special Route 5 Wet in Rally.

Daihatsu Mira TR XX Avanzato R:
Win the Turbo Race of Turbo Sports in the Beginner League.

F090/S Indy Car:
Upgrade a Lancer Evo to the max and complete the Super Speed Way Endurance (about one hour and five minutes). You will win the F090/S Indy Car (great speed and handling) in addition to 200,000 credits.

Ford Escort Rally Car:
Win the Tahiti Maze in Rally.

Ford Focus Rally Car:
Win the Smokey Mountain Rally in Rally.

Honda Civic SiR-II:
Win the Type R Meeting in the Beginner League.

Honda CRX Del Sol SiR:
Win the NA Race of NA Sports in the Beginner League.

Honda NSX Type S Zero:
Win the MR Challenge in the Amateur League.

Honda NSX Type-R:
Win the Type R Meeting in the Amateur League.

Impreza Rally Car:
Win the Tahiti Maze II in Rally.

Mazda 787B:
Win the Like the Wind in the Professional League.

Mazda Eunos Roadster:
Win the Clubman Cup in the Beginner League.

Mazda Miata Roadster RS:
Win the Spider/Roadster Cup in the Beginner League.

Mazda RX8:
Win the NA Race of NA Sports in the Amateur League.

Mazda Savanna RX7 Infini III:
Win the 80's Sports Car Cup in the Beginner League.

Mercedes Benz CLK Touring Car (D2 AMG Mercedes:
Win the Legend of Silver Arrow in the Amateur League.

Mercedes SLK 230 Kompressor:
Win the Legend of Silver Arrow Mercedes Benz in the Beginner League.

Mines Lancer Evolution VI GSR:
Win the Turbo - Race of Turbo Sports in the Amateur League.

Mini Cooper 1.3i:
Win the Lightweight K Cup in the Beginner League.

Mitsubishi Lancer Evolution IV GSR:
Win the Evolution Meeting in the Beginner League.

Mitsubishi Lancer Evolution V GSR:
Win all Golds in the Beginner League.

Mitsubishi Lancer Evolution VI Rally Car:
Win the Evolution Meeting in the Amateur League.

Mitsubishi Lancer Evolution VI Rally Car:
Win the Smokey Mountain II in Rally.

Mitsubishi Lancer Evolution VII GSR:
Win the 4WD Challenge in the Amateur League.

Mitsubishi Lancer Evolution VII Rally Car Prototype:
Win the 4WD Challenge in the Professional League.

Mitsubishi Lancer Evolution VII Rally Car Prototype:
Win the Swiss Alps II in Rally.

Nismo 400R:
Win the Race of Red Emblem in the Amateur League.

Nismo Skyline GT-R S-tune:
Win the 80s Sports Car Cup in the Amateur League.

Nissan Silvia K's 1800cc:
Win the FR Challenge in the Beginner League.

Peugeot 206 Rally Car:
Win the Swiss Alps in Rally.

Shelby Cobra:
Win the Spider & Roadster Championship in the Professional League.

Spoon Sports S2000 Race Car:
Win the Stars & Stripes Grand Championship in the Amateur League.

Subaru Impreza Rally Car Prototype:
Win the Super Special Route 5 Wet II in Rally.

Subaru Legacy Blitzer B4:
Win the Boxer Spirit in the Amateur League.

Suzuki Alto Works Suzuki Sports Limited:
Win the 4WD Challenge in the Beginner League.

Suzuki Escudo Pikes Peak Version:
Win all Golds in Rally.

Team ORECA Dodge Viper GTSR :
Win all Golds in the Amateur League.

Tommy Kaira ZZII:
Win the MR Challenge in the Professional League.

Toyota Celica Rally Car:
Win the Tahiti Challenge of Rally in Rally.

Toyota Celica SS-II:
Win the Altezza Race in the Beginner League.

Toyota Corolla Rally Car:
Win the Tahiti Challenge of Rally II in Rally.

Toyota MR-S S Edition:
Win the MR Challenge in the Beginner League.

Toyota Sprinter Trueno GT-Apex (AE-86 Type I:
Win the Sunday Cup in the Beginner League.

Toyota Sprinter Trueno GT-Apex Shigeno Version:
Win the FR Challenge in the Amateur League.

Toyota Vitz RS 1.5:
Win the FF Challenge in the Beginner League.

Toyota Vitz RS 1.5:
Win the Vitz Race in the Beginner League.

Volkswagen New Beetle RSi:
Win the Beetle Cup in the Beginner League.

Prizes:

Grand Turismo World Championship prize (Beginner League):
Win the Grand Turismo World Championship in the Beginner League to unlock the Toyota Celica GT-Four, Mitsubishi Lancer Evolution VI GSR, Mazda Miata, and Nissan Skyline GTR V-spec II.

Japanese Championship prize (Amateur League):
Win the Japanese Championship prize in the Amateur League to unlock the Mazda RX-7 Type RZ, Mitsubishi Evolution IV GSR, FTO GP Version R, and Subaru Impreza WRX Sti Version VI Wagon.

Japanese Championship prize (Amateur League:
Win the Japanese Championship prize in the Amateur League to unlock the Subaru Impreza Sedan WRX STi Version VI, Chevy Camaro Race Car, Audi TT 1.8T Quattro, and Mazda RX7 Type RS.

European Championship prize (Amateur League):
Win the European Championship in the Amateur League to unlock the Lotus Elise 190, Nissan GTR V-Spec, Gillet Vertigo Race Car, and Mini Cooper 1.3I.

Gran Turismo World Championship prize (Amateur League):
Win the Gran Turismo World Championship in the Amateur League to unlock the Nissan C-West Razo Silvia, Nissan Z Concept car, Toyota GT1 Road Car, and Mazda RX8.

Deutsche Tourenwagen Challenge prize (Amateur League):
Win the Deutsche Tourenwagen Challenge in the Amateur League to unlock the Volkswagen Lupo Cup Car, Volkswagen Beetle Cup Racer, Astra Touring Car, and RUF 3400S.

Gran Turismo All-Stars prize (Amateur League):
Win the Gran Turismo All-Stars in the Amateur League to unlock the Mine's GT-R-N1 V-spec and Raybrig NSX.

All Japanese GT Championship prize (Amateur League):
Win the All Japanese GT Championship in the Amateur League to unlock the Honda Arta NSX JGTC and Denso Supra Race Car.

Altezza Championship Race prize (Amateur League):
Win the Altezza Championship Race in the Amateur League to unlock Tom's X540 Chaser and Toyota Vitz RS 1.5.

Dream Car Championship prize (Amateur League):
Win the Dream Car Championship in the Amateur League to unlock the Mitsubishi FTO LM Race Car and Mazda RX7 LM Race Car.

Dream Car Championship prize (Professional League):
Dream Car Championship in the Professional League to unlock the Toyota GT1, Panoz Esperante GTR, FTO LM Race Car, and F090/s.

Super Speedway 150 Miles prize (Endurance):
Win the Super Speedway 150 Miles prize in Endurance to unlock the Chevrolet Corvette C5R, Tickford Falcon XR8 Race Car, and F090/S