Let's go with Phantom Steed, shall we? 480 feet per round with normal move actions, or if using the Run action it's 960, which is just shy of 110 miles per hour.
If a persisted Haste was used on one, speed would increase to 270, 540 using double move, or 1080 with a regular 4x Run. That's a little over 120 mph. If it was somehow able to get 5x Run it would be 1350 feet per round, or about 155 mph.