Extra Cars
You get extra cars for each world championship you win with variuos cars

Ambulance
Beat the legend Circut level with the pick up truck to get the ambulance

Taxi
Beat the first Circut level

Get Pick Up Truck
Beat the 2nd Circut level with the taxi

Get Street Rocket
Use any car and finish the Novice Championship with gold

Faster Start
Immediately after the Set is announced press A

Get Rag Top
Use the Baja Buggy and finish the Alpine Ridge

Get Baja Buggy
Use the Street Rocket and finish the Destruction Junction with time trial mode

Get Low Rider
Use the Rag Top and finish the Metro Challenge with time trial mode

Get Hatchback
Use the Rag Top and finish the Terminal Impact with time trial mode

Get Woody Wagon
Use the Rag Top and finish the Bayou Run with time trial mode

Get Blue Demon
Use the Rag Top and finish the Seascape Sprint with time trial mode

Get Hot Rod
Use the Rag Top and finish the Sunset Canyon with time trial mode

Get Cop Car
Use the Hot Rod and finish the Midnight Rumble with time trial mode