I never really see much input on this forum for bush flying, so hopefully this helps.
I know guy with an o360 and 31" ABW. He says 29's are probably the right tire.
I have another friend who is on his second pair of 26" ABW. They are more capable than the 26" good years (although not that much for the price difference). I think he wishes he had gone 29's (but won't admit it). The 26's are probably the perfect "side country" tire.
When I wore through my 26" good years, I bought 29's. No regrets whatsoever. I think it's the right size for my o300 on lady legs for actual "back country" flying. Before commiting I obsessed over the added weight (these are 34# per tire with the extra tread) vs the benefit of more angle of attack and lower stresses on the airframe and gear.
To make up for the extra weight, I stripped out the interior (lost about 60#, then added a few goodies back in). The marshmallow landings are
absolutely friendlier on the plane. The added performance from the increased angle of attack was instantly noticeable and I think negates some weight gain -anecdotally anyways. Standing on the tire to fuel is nice, but pushing the plane around got harder.
I still feel like a real #$$@ when landing my 65 year old baby on rocks bigger then 8" though, so try to keep it a little smaller...most of the time. If you are really going to spend a lot of time on big rocks, then the 31's might make sense, but I don't have first hand experience.
With any of them, definitely add a big tailwheel. Especially if you are in the big rocks. If bigger than 26's, then add the double pucks.
Here is a good comparison of the 26" Good Year vs an (over inflated) 29" ABW when I changed over.
Send us some photos of you landing on huge rocks if you go with 31's
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