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.
Why stop there?
Gloves of man (Savage Species) ->? Rapid Wraith (ghostwalk) for x2 speed
Fly like an arrow (dragon 308) for X10 speed
Plus I know there is a reasonable low level Spell compendium spell (edit: cloud wings, druid level 2. I assume you can get a level 3 druid helper) that adds 30 ft, and an item in the MiC (Panther Mask) that gives the run feat.
That's just over 3400 miles per hour, and is greater than mach 4.
EDIT: Wait, only 6 miles? If I can squeeze in another 1680 ft per round, that'll get you there in a single round.
That totals to 300 ft. x5 x2 x10 or 30000 ft per round.