60mph is 1 mile in 1 minute or 2 miles in 2 minutes.
30mph is half that, 1 mile in 2 minutes, so since
2 minutes have already elapsed, he'd already have to
have finished the 2 laps in those 2 minutes it's taken
him to just finish the first lap.  So he can't possibly
average 60mph, even if he could go at the speed of light!
 
Maybe you don't understand that. If not look at it this
way:

Let's figure out how much time it took the car to travel the first
mile lap at 30mph



Now lets figure out how long it would take the car to travel
both laps, averaging 60mph:



There is no way he can average 60mph for the
two laps, because he has already used up the
whole  of a hour just to make the
first lap.

Edwin