I have tested and have been 100% successful in using this glitch in Rallisport Challenge 2. This glitch only applies to Xbox Live. The first step you want to take is to host a game with Xbox Live Rules ON. Then select a race type, track and car. Next is the important part. The glitch is where you can get a car from an other car class even if Xbox Live Rules are ON and the race type is different from the type of car. First, go to GAME SETTINGS and go to RACE TYPE. Then choose a different race type for a car in a different class. When you get to the screen where it says to select a track, instead of selecting a track, go all the way back to the lobby. Then, you can go back to GAME SETTINGS and then CHANGE CAR and it will have a selection of the cars from the different race class. For example, you can use the Peugeot 405 T16, which is a hillclimb car, in a rally race. The hillclimb cars are usually much faster than the regular rally cars, so this should give you a head start and an advantage throughout every single race. Two things to remember though, this glitch only applies to Xbox Live races and you MUST be the host of the room in order for it to work.
Submitted by bmg41 on Xbox Live