I didn't finish the wizard's key dungeon before I headed out, but when I came back to finish it off, it had disappeared from my map. Considering there were at least two artifacts remaining I hadn't gotten yet, it's an annoying bug.
To get 278 mph in the Special Car, I maxed out my engine and raced on the Brazil track. The straight part of the track allows for your to boost your way up to 278 mph. + this comment if it helped!