More parts for your vehicle:
On any race set, complete the last race in that event with a gold medal and you will unlock bonus parts for your vehicle.

Boss Cars in Need for Speed Underground Rivals:
There are 4 boss cars to unlock in the game:

Tank: 1969 Dodge Charger
Triple Dub: Subaru Impreza WRX STi
DJ Wes: Chevrolet Corvette Z06
GT King: Nissan Skyline R34 GTR

These cars are unlocked by beating them in Boss Races and are placed in a special location in your Pocket Garage.

The boss car the game chooses to race against you depends on the level of the race and the car you are using. You need to defete the same boss car in multiple different Boss Races in order to unlock that car. You can choose from any of the Boss Races in Circuit Mode and Street Cross and Drag from Quick Play.

Tank: 1969 Dodge Charger - Race against this Boss Car in a CUP-Level Boss Race using a JAPANESE car.

Triple Dub: Subaru Impreza WRX STi - Race against this Boss Car in a CUP-Level Boss Race using an AMERICAN car.

DJ Wes: Chevrolet Corvette Z06 - Race against this Boss Car in a TROPHY-Level Boss Race using a JAPANESE car.

GT King: Nissan Skyline R34 GTR - Race against this Boss Car in a TROPHY-Level Boss Race using an AMERICAN car.

Nitrous Glitch:
In drag racing if you tap the nitrous button instead of holding the button down your car will accelerate at a faster speed.

Computers and Shortcuts:
In Lincoln Connector an easy way to win is to simply force the computer into the first shortcut of the race. Once the computer is in the shortcut the car will freeze for a period of time. This freezing can even last for up to a lap.