You need to consider what gearing you have currently, and what tire size you have currently to have a good basis for comparison between stock and those two tire sizes. Say that you have 3.55 gears now and 29" tires for example. With that, you can use the following formulas:
If you want to find out what your effective gear ratio with the larger tires given those inputs:
current gear ratio / (new tire size in inches / old tire size in inches) = effective gear ratio
So using the numbers I gave and converting the p-metric tire sizes to diameter in inches:
3.55 / (31.6 / 29) = x
3.55 / 1.0896 = x
x = 3.258
So basically, when you switch from 29" tires to 265/75 tires, your gear ratio effectively dropped from 3.55 to 3.25. If you went to a 285/75 (32.8" diam) then you would drop even lower to approximately 3.13.
To get back to the stock gearing (3.55) by regearing (assuming you went to a 265/75), here's what you do:
(new tire size in inches / old tire size in inches) x existing gear ratio = new gear ratio needed
(31.6"/29") x 3.55 = x
1.0896 x 3.55 = x
x= 3.868
So to get back to an effective gear ratio of 3.55, you would install 3.868 ratio gears (in theory). I don't believe they make anything like that for our axles so you'd have to decide between 3.73 and 4.10 as far as regearing. The upside is you could just swap in a set of 3.73 diffs from a salvage vehicle and save some money (as long as they are in good shape). If you went with 285/75's then 4.10's would be more appropriate to get you back to an effective gear ratio of 3.55.
To see what effective gear ratio that will result in with your size tires and a given gear ratio, you use the first formula I listed, but you use the planned axle ratio instead of the current one you have:
planned gear ratio / (new tire size in inches / old tire size in inches) = new effective gear ratio
So I'll use 4.10 gears and 265/75 tires for example:
4.10 / (31.6/29) = x
4.10 / 1.0896
x = 3.76
So 4.10 gears with 265/75 will give an effective gear ratio of 3.76. This would feel a lot more quick off the line than stock, but not be bad at highway speeds either. It would also be helpful since you have the 4.0. I'll also add that I had this very combo on my last truck and it helped a LOT having those higher gears, especially when I was running mud tires.
Hope that helps with the gearing part.
Also you might consider that a 265/75 will just fit in the spare area...I don't think a 285/75 would go in easily, if at all. May not be important to you, but since I have a 4wd and don't want to get stuck somewhere with a flat tire and and undersized spare, I have limited myself to 265's.
