Nitro Bike in Mario Kart Wii? It's from Mario Kart 8

The Nitro Bike does not exist in Mario Kart Wii. It debuted in Mario Kart 8 (Wii U) and returns in Mario Kart 8 Deluxe (Switch) as an inside-drift bike for medium/heavy characters.

Closest Equivalent in Mario Kart Wii

The Flame Runner (large bike, heavyweight class, inside drift) is the functional analog—high top speed, strong drift, favored by competitive players for time trials.

Full Vehicle List Access

Check the in-game "Vehicle Select" screen after unlocking all Grand Prix cups. Every kart and bike for every weight class is listed there with stats (Speed, Weight, Acceleration, Handling, Drift, Off-Road, Mini-Turbo).

If you're playing MK8/Deluxe, the Nitro Bike unlocks via random coin collection (every 50 coins = random part unlock).