Sega brought developer Sumo Digital's kart-racing (and flying, and boating) game to iOS today. It will make the jump to Android "soon." The app costs $5.
Racing Transformed launched first for PlayStation 3 and Xbox 360 and then made its way to Wii U, PlayStation Vita, PC, and 3DS. I recommend giving it a go if you haven't already. The mobile version introduces a new racer: Shenmue’s Ryo Hazuki and his vehicle, an arcade cabinet.
The original Sonic & Sega All-Stars Racing achieved 12 million mobile downloads.