How do you unlock the Sprinter Kart and Mii Outfit B in Mario Kart Wii?
Solution 1:
Unlocking the Sprinter requires you to unlock 24 Expert Staff Ghosts. Unlocking the Mii Outfit B requires you to unlock all 32 Expert Staff Ghosts (one for each track).
You unlock the Expert Staff Ghosts by beating a specific time on Time Trials for each track, while racing against the Normal Staff Ghost.
There are many online links that show the times you need - generally if you can beat the Normal Staff Ghost by 3 or 4 seconds, you will unlock the Expert Staff Ghost.