To do this, you must have one of these cars in your garage: Mitsubishi Eclipse, Acura Integra, Honda Integra, Mazda RX-7, or Mazda RX-8 (modifications/upgrades are OK and highly recommended for this, specifically those that increase speed & acceleration to it's limits) AND have reached Level 5 (Level 10 if using the Mazda cars). Enter either the Amateur "Tuner Face-Off" or "Rotary-Engine Challenge" events, change all difficulty levels to the hardest levels, and choose the "Sunset Peninsula Speedway" race. If you can win the race, which should be easy if your car is a higher class than the CPU, your bonus for difficulty can add up to 65-75% or more of the 1st place prize to your earnings, which could add up to 10,000-15000 CR per win. So if you win the 7000 CR race, and bonus is 65%, you could earn at least 11,000 CR per victory after bonuses & penalties (which should be 0 if you drive carefully). This short race (2 minutes or so) is a great way to quickly earn credits and move up levels, especially early on in your Career. This works on any series with Sunset Peninsula Speedway, but these two are the easiest to do it on.
First, go buy the VW jetta. Now buy all the engine upgrades EXCEPT for a new engine. Now once you have all the engine upgrades, buy a new engine. Then install your old engine and sell the new one. You're actually selling it for 32 thousand credits! Just keep re-buying the engine, and selling it for large amounts of credits.
First, go buy the Corvette Z06. Now buy all the engine upgrades (the icon on the left) EXCEPT for a new engine. Now once you have all the engine upgrades, buy a new engine. Then install your old engine and sell the new one. You're actually selling it for more than you bought it! Just keep re-buying the engine, and selling it for large amounts of credits.
Create a new profile and put in tEAm4za as the name to get 900,000,000 credits
Enter nOsLiW as a profile name to start on level 50. Note: code is case sensitive.
Type nOsLiW Case Sensitive to be lvl 50 on career and all cars unlocked in career and free run and so on