I think The Flash could outrun Superman. But Superman can fly. So depending on how the racecourse is set up, that would determine the winner.
But Superman is pretty fast, and the longer he has been on Earth, the more his powers have grown. For example:
Originally, Superman was just faster than a speeding locomotive. Now he is much faster. He just may give the Flash a run for his money.
Originally, Superman could just leap over tall buildings with a single bound. Now he can fly.