Quote Originally Posted by GreatWyrmGold View Post
That assumes a constant speed. If the caster spends any time accelerating or decelerating, the peak speed is going to need to be higher. If they spent three seconds accelerating at a constant acceleration and three decelerating, the peak speed would be 1200 mph, and the acceleration would be just over 18 G (thanks, Wolfram Alpha!). That's not necessarily lethal on its own, if you're accelerating in the right direction, but air friction would probably rub you the wrong way.
It's not unreasonable to assume the spell moves the air around you as well. That way, you still have to deal with the acceleration issues, but the effects of air friction would be lessened, if not negated.