Sometimes you can get 3 firsts and get
almost dead last (like 10th) and still get first overall. I've had it happen to me because of that

ing grumble volcano and those

ing blue shells.
Unlock Funky Kong by unlocking four expert staff ghosts. He is the fastest rider in the game. Also has one of the most annoying voices.

Use the Flame Runner + Funky Kong. FR has nice handling and great speed considering it has decent stats in the other categories as well. You unlock expert ghosts by beating regular ghosts by a certain amount of time. Just try on cources you are good at/are easy. For example, the ghost on Luigi Circuit is a pushover and I unlocked the expert ghost on my first try. Same thing in Maple Treeway, except I think it's just because I'm good at that track.
Expert Ghost unlock guide:
http://boards.gamefaqs.com/gfaqs/genmessage.php?board=942008&topic=42526151