Start with 900,000,000 Credits:
Enter tEAm4za as your profile name.

Start at Level 50 and All Cars and Tracks Open:
Enter nOsLiW as your profile name.

Drive on an Empty Tank of Gas:
With a manual transmission car, when you run out of gas, you can shift back and forth between gears each time you do this you gain 1 mph.

Free Cash:
Note: This can only be done with a unpatched game that has not yet gone onto Xbox Live.
Buy a WV Jetta VR6 GLX. Then go to the engine upgrades menu and buy every upgrade level in every section there, except the upgraded engine. Then buy the other engine for the car, install the stock engine, and sell the non-stock engine. You will then be able to buy the other engine for around 2500 and sell it for around 45,000. Rinse, wash, repeat.

Faster Free Cash:
Note: This can only be done with a unpatched game that has not yet gone onto Xbox Live.
Buy a Chevrolet Corvette and do the same engine upgrades process in the cheat above. This time, you will invest 32,000 CR and make back more than 101,000 CR.

No Prize Car Bug:
This game has a bug where if you do not place 1st the last race of the championship you will not be awarded the prize car, regardless if you have the most tournament points. So be sure to keep trying that last race until you com in 1st place so you get your prize.

Copy Cars between Profiles:
This will work for as many profiles as you have, but 2 is the minimum and will work if you buy a car off someone in Xbox Live but do not have it unlocked. Ok: Profile A has the car you want to copy and the other is Profile B. Load B, then select Profile to Profile in the Garage Menu. Select Profile A, find the car, and buy it from Profile A. You will now have the car in both Profile A and B. Now load Profile A and buy that car you just copied so that you can have 2.

Free Window Tints:
Scroll over to the color you like and hit B. Your window will be colored and you just saved 500 CR.