Someone with Mythic Speed and athletics of 1 can move ten zones. 1 for his base athletics, 6 for twice the mythic speed bonus (it's doubled when sprinting), and another 3 for a free supplemental to move 3 zones. While sprinting he has an athletics defense of 7 (epic). Now, he may also set aside the doubling of the athletics bonus when sprinting to keep up with vehicles. I.e. he can simply declare he moves that many zones regardless of his actual athletics (which is only then used for defense).
As for the Flash, he probably has several upgrades heaped on top of plain Mythic Speed. Something like Kinkaid's mobility stunt to move a further 2 zones as a supplemental (total of 5), acrobat stunt for better ranged defense, an equivalent dodge stunt vs melee, a -1 refresh power to move in surfaces of any kind (such as ceilings and the surface of the sea), a -1 refresh power to allow maneuvers and blocks with athletics (similar to Incite Emotion, only physical), and finally a -1 refresh power that, for a fate point, allows him to reduce the time scale required to perform a physical task by his athletics or to use his athletics for an attack.
He also should have Physical Immunity to friction, acceleration and high-speed impacts; for him he is not fast, subjectively - the world is slow.
Yeah, -15 refresh for all his abilities seems about right.