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It's 2270 at 50lbs I believe. And my x is otherwise stock so the curb weight is right around 4000 lbs I think?
You use GVWR, not curb weight.
So a 5,200 - 5,400 lb GVWR is for most Xterras.
If your GWVR (On placard) is 5,200 lb for example, that means that ~ 35% of your GVWR is 1,820 lb. (35% is used instead of 25%, because loading is dynamic, and loads go up when braking, cornering, etc)
To be able to support at least 1,820 lb, if the tire is spec'd at 2,270 lb at 50 psi, = ~ 40 psi...not 30 psi, which is now under inflated.
To get all that, which is a MINIMUM, you would divide 1,820 by 2,270 lb, to see what percentage of the max weight, 1,820 lb, is.
1820/2270 = 80%
So, you then multiply the max PSI (50) by that percentage, yielding 40 psi.
So, 40 PSI is the bare minimum, and, you'd ADD about 5 - 10 psi to that if you are driving at hwy speeds a lot, or towing, etc.
That means you got, marginally, too low a load range.
You don't want to go over ~ 80 - 85% of the max load for your tire's psi setting, as that's outise the tires sweet spot, performance wise.
Typically, 60 - 85 % is about right overall, depending upon the tire.
So, its not unsafe, just loaded to ~ 100% of capacity...which is hard on tire life, so instead of getting ~ 90k miles you might get 50-60k out of them for perspective.
If not too bouncy, etc, you should be at ~ 45 - 50 psi or so, but not less than 40 psi...as that's the bare minimum.
OFFROAD - you obviously are going much lower, and for that tire, ~ 15-20 psi will probably be what you find useful.
For a given day/terrain, just park the tire on a rock or stump/log or whatever, and air down until the tire "wraps" or grabs the rock...and then stop.
You want the tire's contact patch to CONFORM TO THE TERRAIN, but not be a flat tire...so you have sidewall height, but the tread can still deform into the terrain's nooks and crannies, etc...to maximize flotation and traction.
Notice the sidewalls still have height, and the way the tread is gripping the stump or rock?
Notice how the tread elements are actually
pinching the texture of the rock in the close up?
That's what you air down to get...so just put the tire on something TO wrap, and air down until it wraps it....and that's the PSI you want.
The tire in the above pics are LTB's, 33 x 13.5/16's...so 33" diameter on stock 16x7 rims, so the full sidewall height started at ~ 8.5".
Some of that, but not all of that 8.5" of sidewall height, was used to get that sidewall bulge/tread wrap and flotation action.
The more you air down, the larger your contact patch gets, up to a point, and then it can't get larger and you went too far. Also, airing down lowers your diffs, etc, all the lower parts that only the tire height is giving clearance for.
This is one of the reasons you got larger diameter tires FOR typically, was for added ground clearance...which airing down eats into...so, finding YOUR rig's sweet spot is a good idea.
As a side note, you could have fit 32x11.5/15 on there instead of the wee 31x10.5/15's...and got a 2,535 lb at 50 psi rating...so it would be at ~ 35 psi minimum for the same load...instead of 40 psi...which would have left a little fudge factor to reduce wear and tear, ride a bit smoother, and give an extra 1/2" of lift/clearance.