235/75 R 15, work pretty well. Though I did have to put a washer behind the steering stops. They would rub a bit at full turn.
I´ve got a set of 30 X 9.50 (actually 29.5 inches inflated) mud treads on the other XJ. Also with the washer behind only the left steering stop. You have to bounce them around pretty good, before they hit anything, then it´s only a little.
225/70 R 16 seem to work really well, fill up the wheel well and require no washers behind the stops and have never hit any part of the fender well yet.