To get a Class C car fully upgraded you must go to "ARCADE MODE."
~In ARCADE MODE you can buy the fastest car of each class, choose any you like. I chose the 'Nissan 240SX'. Once you have your car go buy a 'McLaren F1' to help aid you, ill explain later.
~Go to race on the menu. Click and drag the meters bar to the farthest meter eg."1500m". Then click and drag the difficulty bar till you see the words ricer appear. Now click Race...........Race with the McLaren car and win about 12 times, just enough to upgrade your Performance. Go to performance after your 12th win and add one point to each item so its even throughout. Now with the McLaren upgraded you can now race people for two points a race istead of one, so to do that go back to race and drag the meter bar as far as it can go and then o the same to the difficulty bar EXCEPT stop when it says "Pro Tuner".
~ Now race over and over till you get about 36 performance points(*keep checking how many you have by going to performance after a couple races*). Do all of this with the McLaren. Once you've gotten 36 or more performance points, head to your garage, drive you CLASS 'C' vehicle(whatever it may be), and go to performance then upgrade it completely till all items contain 3 performance points each.
~ Heres the hard part, take that car, and go to an easy race, so basically go in and race right away, dont change anything. When the race starts, you will see the crowd, if you roll your mouse over the crowd sitting in the bleachers nearest to the wall, the mouse will change from an arrow to a hand at some point(*meaning a clickable button is here*). Click that area of the crowd and "Wala!" there you have it, it takes you straight to "TUNER MODE" in the touney's section, all you have to do is click Garage and everything is still the same.
~THERES ONE BUG THOUGH, YOU MONEY IS UNDERFINED, SO LETS FIX THAT...........................If you save the game in the upper right hand corner and reload the page, or go back a page and re-select the game, when it loads up you can choose either "ARCADE MODE", "TURNER MODE", or "ONLINE MODE", WELL DONT CHOOSE ANY OF THOSE, AS A MATTER OF FACT LOOK DIRECTLY UNDER THOSE, THERE IS A CLICKABLE TEXT THAT SAY CONTINUE IN BIG LETTERS, CLICK IT NOW! and it will load your game and you should have all your info in the cars you had and also the word undefined should be gone now, MEANING YOU CAN NOW BUY UPGRADES FOR ANY OTHER CAR CAUSE NOW YOUR MONEY'S AT INFINITE, MWWWHHAHAHAHAH!
*Thank you very much, and have a good day. I also want you to know that I actually discovered this glitch or malfunction in the game about 4 years ago, so I didnt get it from anyone else, lol, see ya.
~QuepiD~