Here is some interesting info taken from Eurogamers review:

Bearing that in mind, it’s odd that Nintendo has structured Mario Kart Wii the way that it has. The entry-level 50cc Grands Prix are for karts only, and the 100cc medium-difficulty races are exclusive to bikes, while 150cc allows both types of vehicle. As with Mario Kart DS, there eight championships, with sixteen new tracks and sixteen reprisals from former versions of Mario Kart, all the way back to the SNES original. Only once you’ve fully completed all eight will you unlock the chance to race 50cc and 150cc with both bikes and karts..