Successfully complete each of the four parking levels to unlock an additional parking level. Successfully complete the bonus parking level to unlock a sixth parking level. 

Nippon Maru:

Successfully complete arcade mode with all four characters: 

GameShark Codes 

Effect Code
(M) Must Be On EC8935C81440C69C
Press R1 For Max Cash 0CCA0E261456B80C   1CA7B8C414BE598C   0CCA0E261456B80C   1CCD709414BE598C
Press L1 For More Time D05F8D020000FBFF   103037A40000EA30
Press R2+L3 For K.I.T.A. Turbo 0CCA0E261456B20A   4CE4C6AE14562555

It's absolutely critical that you don't turn into a corner too early (even more so than with car racing sims). If you do, the bike will understeer heavily to the outside, and you will be completely offline for the straightaway that follows. By being patient and waiting for the proper turn-in point, you will be able to rocket out of the turn with considerably more speed.

If you have the traction-control aid turned off, your bike will slide out from under you every time you so much as touch a blade of grass on the outside of a corner. Oddly enough, you can put your wheels on the grass or dirt located on the inside of the turn without suffering any
such dire consequences. Use this to your advantage as much as possible, but don't cut the corner too much, or you will bring on a stop-and-go penalty for shortcutting.

If you use a multiple-axis joystick with a twist grip and a separate throttle control, you can map your  rider's lean-and-pitch movements to these axes for more hands-on control and a somewhat greater challenge.

Lizard Tail:  Successfully complete the game with all five trucks (Asphalt Cowboy, Stream Line, Long Horn, Highway Cat, and Nippon Maru).

Bonus trailers:  Successfully complete arcade mode with all four characters to unlock two bonus trailers in score attack mode, versus mode, and arcade mode (if the Nippon Maru is selected).

Bonus parts:  Go to arcade mode and select any truck. Get to your destination ahead of your rival , then complete the bonus parking mini-game to win bonus parts. Note: This works on any destination.

Shifting:  For Highway Cat and Stream Line, shift up when at about 50 mph.

Slip stream:  When you are behind a truck, your size you will get a slip stream. This will allow you to get a small boost to get in front of the truck.