I've used the 18" wheels (SE Td5 2a) to do Flinders Ranges, Oodnadatta, Simpson, Birdsville, Innaminka, Vic High Country, etc.
They did very well with std size 255/55-18 tyres.
Only weakness was the tyres themselves.
I used Nitto Terra Grappler and Grabber AT2 tyres but couldn't get them in LT construction - thus they were a bit susceptible to rock punctures if travelling too fast but otherwise great on all surfaces.
After the Simpon trip I went to 8" wide 16" Land Rover alloys and BFG AT and am very happy with all aspects: off road, rock protection, on road handling (does have ACE). Went for 255/70-16 which gives a little more diameter and still fits on the rear door, no rubbing or issues anywhere.
However, the 'new' BFG AT KO2 comes in a 255/55-18 with LT and to be honest, I would've preferred to just do that, rather than the cost of buying the new wheels and having the old ones lying around the place (gotta chuck them out at some stage...).
cheers,
bidds
