http://www.expeditionexchange.com/disco1spare255/indexmain.shtml
Here is the text, go to the link for the pictures:
Relocating The Spare Wheel Carrier For 255/85-16 Tires
by Ho Chung
The spare wheel carrier on the Discovery Series I can take up to 235/85-16 tires without any modification. There is actually the infamous "Spare Wheel Rule", something along the lines of "don't fit tires larger than what your spare carrier can handle." To keep within this rule yet fit larger tires, we must modify the spare carrier.
There are adapters available that enable you to fit larger tires to the spare carrier. The disadvantage of these adapters is that you end up with a spare wheel that sits further away from the door panel, adding more weight and stress to the already heavy door. So I've decided to relocate the carrier. I didn't think it would be this easy. Plus, I always liked to have the heavy spare wheel sitting on the rear bumper.
First you must remove the rear door liner, being careful not to break the red plastic clips that hold the liner to the rear door.
The above diagram gives a rough measurement of the new holes required to fit a 255/85 spare wheel. The new holes are roughly 3/4" below the original holes. The best way to locate your new hole location is to remove the spare wheel carrier from the door panel, bolt it to the spare, rest it on the bumper, and mark the new location for the new set of holes. I did that from the inside of the vehicle with a thin pencil through the holes in the door.
Once you drill the new holes in the spare wheel carrier, finish the bare metal to your liking (I used black spray paint). Then simply bolt on the modified carrier.
Note that the bottom holes fall on the curved part of the carrier. You'll have to smash that part flat in order to drill a hole in its intended location.
The upper part of the carrier is curved to match the curvature of the door panel. Hopefully your 255's are the same size as my BF Goodrich 255's, because the curved parts fit there perfectly.
One last detail is to pull out the wiper arm from it's stud, and rotate it clockwise so that it clears the carrier.
With the relocated carrier, I now have the spare wheel sitting on the bumper. This alleviates some of the weight off the door while bouncing around out there.