There are several sets of wheels you can unlock for your kart in in Mario Kart 7 for the Nintendo 3DS, most of which are unlocked by collecting coins in Grand Prix mode.
The wheels unlocked with coins will show up in random order.
Unlockable Wheels With Coins
Red Monster Wheels
Wooden Wheels
Unlockable Wheels From Other Methods
Golden Wheels – Finish all Cups on 50cc, 100cc, 150cc, and Mirror Mode with a “1 Star” or higher
Check out my other guide if you’re looking for the full list of unlockable kart parts in Mario Kart 7.