First of all, I don't know if this has been reposted, I searched and couldn't find anything.
You need to go into Icecrown and get to Death's Rise, and do the quest chain there up to the quest 'You'll need a gryphon'. When you get this quest you will need to get to Onslaught Harbor and kill a gryphon rider and take the reins. Now when you use the reins you jump onto a gryphon that flies at 100% speed.
As long as you don't turn the quest in you can use this forever. There are also some benfits because it is counted as a vehicle : You can't get knocked off,you can fly indoors, if something attacks you it hurts the gryphon, not you,you get a parachute if you jump off, and it has about 1/2 the cast time of a normal mount, and if you're horde, you might like this even more as its a gryphon!
The mount only works in Icecrown remember.
EDIT:It runs at 100%. I found this out just about 10 min ago as I was doing the quest myself As in I found this out while I was doing the quest