$999 by hitting bonuses:
Hit a bonus head first, then press L1 R1 X. You will get the maximum amount of $999. Note: This must be repeated each time.
Harry Buck.

Bonus parking levels:
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:
Hold X and press Start at the title screen. Keep X held until the mode selection screen appears. Choose any mode to drive the Nippon Maru truck. Alternately, successfully complete arcade mode with all four trucks.

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).

Hint: 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.

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

Hint: 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.