Superman dies in the sun, like you die when you have a protein bar...
Flash is the fastest man alive...
Now if you're talking sundip... like superman 1 million's sundip.. maybe he'd be faster than the current flash. but... i don't think that as far as comics are concerned superman is ever supposed to be faster than the flash.
Ie. smallville. Bart was racing clark, and ran backwards alongside clark for a while, while clark was giving it his all. In the end Bart took off like nothing agianst clark. Thats how a real race with flash and superman should go... Oh dear god I have all of season 5 on dvd at home.. must go watch...
I have a book with all of Superman and Flash's races.
Superman always comes a very very close second. When they're running, there's always steps in between them.
Supes will never be able to match Flash running through an atmosphere as his speed would cause destruction to the environment, whereas Flash's speed doesn't do that as he is protected by the speedforce.