The skinny vs fat tire debate has raged for decades...about when
they started to use use inner tubes instead of a solid rubber strip
around those wooden wagon wheels...
The concensus is:
That a skinny tire has better contact
pressure (Same weight suppoorted, on a smaller contact patch)
That a skinny tire will sink down...and IF there is anything within
an axle depth to bite...it can bite it and grip.
That a fat tire has worse contact pressure (Same weight, distributed over
a larger foot print/contact patch)
A fat tire provides flotation, preventing you from sinking down.
(Good if there is nothing within range to bite down there anyway...)
So - There is no one right tire ratio for every scenario....
Skinnies work better when contact patch pressure wins,
Fatties work better when contact patch size wins
The tricky part is to calculate the contact pressure you need for adequate traction,
once you hit the point of diminishing return...and that is VERY situation specific.
Some tire treads need more pressure than others for the pattern to bite/grab or self clean.
The heavier the rig, generally the fatter the tire can be w/o loosing too much contact pressure..
I find personally that rocks are more navigatable around 20 psi than 10 psi...it gives
more diff clearance, and provides enough pressure to let the tread blocks squeeze
the rock edges for better bite...Mud on the other hand seems to work better as low as you can go w/o
blowing the beads...the RPM needed to clean out the lugs spins the tire out to full diameter anyway,
the exception being some kinds of mud where you need a stiffer bladed lug to get a grip...To clamber up a hill with loose gravel/leaves, etc....the contact pressure seems more important than the size of the patch, so the skinny tire will win...
I'm guessing, w/o Xterra specific empirical evidence, that an 11.5" or so section width/~285 mm sized tire is about right, too much more and the turning radius will be enlarged, tire tuck looks to be at the limit, and the contact pressure will be nearing the point of diminishing return (IE: Hill climbs soon generating wheel spin, etc...)
The stock tires on mine are 29 x 10/16's (Grabbers) = 255/65/16...and it looks like around an inch or so clearance before the torch/dremel (whatever) gets fired up...so a 285/70/16 would be optimal..involving the lose of the mounting point for the mud flaps, and the same distance towards the front, maybe a little adjustment under the rear wells too, like the gas filler area under there...etc... I think. (Theoretically...that size may not be on the shelves anywhere...but in a 15"....around a 32 x 11.5, assuming the common undersizing of the nominal 32" diameter.
You just need to know the limiting factors involved with YOUR terrain.
Does this help at all?
- Lance