The stock wheel size on a Ford Expedition is
17x7.5
The stock tire size is 265/70/17.
With that said, the largest tire you can mount on that size wheel is
285/70/17
I just recently installed tires on our truck (08-19-06) and ran into a problem.
I had purchased the tires on tirerack and when I went to the shop for mount and balance, they said they could not do it. I asked why because a 285 tire will fit on a 7.5 inch wide wheel.
According to the service rep, most of the major chains (Discount Tire, Big O Tire, etc) will not mount a 285/70/17.
He mentioned something about that size tire on that size wheel failed some kind of bead test. However, I've known plenty of people who have ran this size tire on this wheel with no problem.
Don't know if what the service rep was saying was true, so I left and went to a smaller mom/pop independent store and got the wheels mounted.
I'll report back if there are any problems, but I doubt their will be.
In any case, you may run into the same problem that I did.