Dude.
General Tire offer several size options in an At2 pattern which has great response in other threads from other vintage machines from our Land Rover clan & at least this All-Terrain tyre comes in the factory standard fitment of 255/60R18 too !
The have listed on their current web-site
www.generaltire.com four other options as 285/60R18 (31.5") - 275/65R18 (32.1") - 265/70R18 (32.6") & 275/70R18 (33.2"), but just check the speed rating of your beast as this could effect your insurance ?
Codynt has the standard 18x8 inch wheels on his beast which look and fit great with the 32.1" Coopers, yet as littlehandegan confirms his Disco3 rims are 18's & if I'm not mistaken these are 18x7.5 inches-this will effect which tyre size is appropriate.
Typically where a wheel is half an inch narrower the same size tyre will sit 0.5" lower in height than that specified O.E.M. measurement and 'bulge' more at the same pressure, compared to the factory 18x8" example which would sit broader and taller respectively. The resultant clearance issues of the shock or suspension components comes into conflict here also.
So as you can see from the theoretical table stated above, by opting for the 265/70R18 equivalent brand of manufacturers rubber, this should give a fitted tyre on a 7.5" wide rim static height of 32.1" with a cross-sectional width of 10.2"[259mm].
Hope this helps.
Cheerio,
Vinniman.
'88 Highline
Perth, W.A.