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.
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.
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.

• 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.

• 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.

• 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

• Boss car warning
When considering buying a car, do not buy any of the cars that are used as Boss cars. If you buy one, you will end up having two of the same car. Your garage can only hold 20 cars, and there are 24 cars total. The four Boss cars have their own separate garage and do not take up any of the twenty slots. You cannot get 100% completion if you buy any of the cars used as Boss cars.

• Bonus parts
Successfully complete the last race of a set with a gold medal to unlock bonus parts.

• Avoid losing control
Before hitting a wall, press R1 to help avoid losing control.

• Helpful momentum
In the Nitrous Run Race, if your time is about two to four seconds from expiring, aim the car at the gate and use all the nitrous remaining. Even if the timer runs out, your car will keep going enough without the gas and may pass the gate. If so, the timer will start again and your nitrous tank will be refilled, allowing you to continue with the race.

• Getting driver points
You can earn all the upgrade points in an event by winning it on the Bronze and Silver difficulty settings. However, to get all of the driver points you must win all of the races on the Gold difficulty setting. Check the "Driver Status" screen in "My Underground" to see your current driver points and driver rank. Maxing out your driver rank will unlock secret bonuses.

• Fast Mazda RX-7
After getting the RX-7, put the following upgrade parts on the car to obtain maximum speed: Aeromax spoiler, Beast Carbon Fiber hood, Gills roof scoop, and Max 1 body kit. Also, make sure you have all of the performance upgrades unlocked and ready to install. You can get over 210 mph with this setup and win a race by 42 seconds.

• Car selection
If you are having trouble figuring out which cars to use for what event, cars with high top speeds/acceleration are good for drag races; cars with good controls/handling are best for drifting; and cars with a balance of the three are good for normal racing.

• Nitrous runs
On the nitrous run levels, let off the gas through tight turns. Otherwise, you will hit the wall and flip your car.

• Ride invisible wall
While driving, hit a turn too hard to fly up and over the railing. There is an invisible wall that you can ride. If you ride it correctly, you can land back on your wheels.

• Drive through doors
On the Bayside Run course, there are doors that you can go through without them moving. Drive over 175 mph and have NOS on when you are about to hit the doors. You will disappear and go through it to the other side without moving the wall. When you enter the sidewalk sale area, the right front door does not seem to work but this glitch can be done with all others.

• Music
To listen to the music that you have encountered, go to the main menu and select "EA games Pocket Trax". Here are the song that are in movie mode:

Soulwax: "Any Minute Now"
The DONOTS: "We got the noise"

• Car list
The following cars are unlocked for purchase in order:

Volkswagen Golf GTI 2.0L: $22,000
Ford Focus ZX3: $18,000
Mazda Miata MX-5: $19,000
Dodge Neon: $17,000
Nissan Sentra SE-R SPEC-V: $21,000
Toyota Celica GT-S: $23,000
Nissan 240SX: $30,000
Mitsubishi Eclipse GSX: $32,000
Mazda RX-7: $34,000
Mazda RX-8: $48,000
Pontiac GTO: $46,000
Subaru WRX STi: $46,000
1969 Dodge Charger: $44,000
Toyota Supra: $52,000
Nissan 350Z: $54,000
Mitsubishi Lancer: $51,000
Ford SVT Mustang Cobra R: $56,000
1967 Ford Mustang: $65,000
Nissan Skyline R34 GTR: $70,000
Chevrolet Corvette Z06: $72,000