Go to the car select screen and press Up, Up, Down, Down, Left, Right, Left, Right, Square, X. If you input the code correctly, the ultimate car, Furasshu-sama, will immediately appear on the screen. If your not fast enough you'll be taken to another screen upon pressing x.
On Bottom Of Your Car:
Go to Create Car Put In Up Down Left Right Up Down Left Right in 10 seconds fast. Lights Just Work At Night
300 MPH car:
Go to the car select screen and press Circle, Square, L1, R2, Right, Left, Up