eEwwww! I love tire threads!!! Super show and tell. I realize that this thread is requesting tires that will work for a Land Rover Series II, IIa, III but here are good tires and my 0.02¢ experience on these that I've used since 1980. Hopefully you'll find this interesting or at least entertaining eventhough the tire designs must be as old as me by now! Listed in order of what worked best for me, what condition and what vehicle they were on.
Best Mud, Deep Snow and overall good on the highway tire is ...
Goodyear XT terra tire, 31x11.50-15
Also available in 31x15.50-15 (lugs wrap around the side). I've used this on my old 1982 Toyota 4WD longbed pickup (hilux) in loamy wet spring mud near the Rubicon, deep snow, Scott's Creek sand (north of Santa Cruz, CA), granite rocks, SUPER DEEP sticky mud, deep river fording, dry dusty conditions and experienced hot tarmac on busy California higways to and from Lake Tahoe = had to make emergency stops with them. These are not as noisy on the road as you would expect - they just plain work. We routinely used to get 20,000 miles out of a set running mostly on highway. They worked in the rain too. These are NOT tread lightly tires, they dig big time. We had to purchase these through a farm account. If you can find a set, buy them. Might be out of production, my pick of the day.
1982 Toyota 4WD longbed, 18RG twin cam 2.0 liter, dual Mikuni carbs. Goodyear XT terra tire. (sorry for the lousy photos, scanned from 35mm slides)
Firestone ATX
Another good choice for mud, just different than the Goodyear XTs. Not as good in the rain, too much flat un-siped tread surface. Might be out of production.
1980 Isuzu 4WD pickup, Firestone ATX.
BFG Mud-Terrains
Everyone knows how well these work. They are noisy on the highway, and work well off road in Telluride, CO Black Bear Pass loose wet rock conditions. In Vermont mud (similar to Rubicon spring mud - if there is such a thing?) they work really well. About the same as the Goodyear XTs on the highway. Good Choice.
1994 Defender 90 NAS soft top. BFG Radial Mud-Terrains.
Mickey Thompson Baja Belted
AWESOME in the sand and gravel. 2 Fiberglass belts, that allow lower than normal air pressure levels. I routinely ran these down to 8-12psi. Good for dry conditions. Ok in the mud, but they pack up quickly and are harder than normal to clean. Slick in the rain, must have been the harder or different rubber compound. Can be run either direction.
1996 Toyota Tacoma 4WD extracab, 3.4ltr v6 5spd. Mickey Thompson Baja Belted.