If you get directly behind another sufficiently large truck (not a van or car), you'll enjoy a small amount of additional acceleration. Driving in silpstream is good, but don't stay in it for too long or you'll hit the guy in front of you.
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.
Play as Nihonmaru
Successfully complete arcade mode with all four characters to unlock Nihonmaru.