Based on figures in my earlier post:
Increasing from a 30 x 9.5 to a 31 x 9.5 (245/75R16) would increase my floation 4%.
Increasing from a 30 x 9.5 to a 30 x 10 would increase my floation 4%.
Increasing from a 30 x 9.5 to a 31 x 10.5 would increase my floation 14.5%.
I know from prior years experience with various Jeeps, tire sizes, and amounts of gear that I like enough floation to get me by on sand (when aired down), but little enough floation to let me have good grip on Winter roads without having to air down.
I got that ^ beautifully from a stock 96 XJ with 30 x 9.5 tires.
But my 99 XJ recently weighs 4% more due to gear added to it (skid plates, light winch mount and light winch with syn rope). So now 30 x 9.5 tires no longer give me enough floation for sand, even aired down. I'd sink to much. It'd be great on Winter roads, but poor on sand.
I want to add 4% floation back to it with larger tires so I'm back at the same contact pressure I had with a stock XJ on 30 x 9.5 tires. I know from experience that is low enough contact pressure to work on sand, and high enough to work good on Winter roads.
So I was trying to decided whether to upgrade tires to 245/75R16C (31 x 9.5) or 31x10.5R15 tires. I want to add as close as possible to 4% to my flotation, or to say that another way, I want to reduce my contact pressure 4%.
So after doing the math, I know that 31 x 10.5 would decrease contact pressure (increase flotation) to much, while 245/75R16 tires would be perfect (for what I want). I already suspected this intuitively, but now I've confirmed it mathematically.
Warning: I'm notorious for making a post, then editing same post several times, adding info each time, or correcting typos.