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Discussion Starter #1
I just put a set of 31/10.5R15 BFG KO2s on my 2000 xterra. The shop recommended a street pressure of 40lbs or so for those tires which is considerably higher than the stock pressure. My question is what should I air those down to for off-road use?

Oh by the way, a lot of people have reported running 31s on a stock 1st gen with no clearance issues but that was not my experience with these. Maybe it's just the way the tread wraps onto the side on these KO2s but I had to ditch the mudguard and trim off that lip because they were rubbing pretty bad.
 

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Discussion Starter #3
I do 30 street 15 off road.
What tires and wheels are you running? 30 seems pretty low to me for the BFGs. I've been running mine at 40 (where the shop recommended) for a few days now and they feel pretty good on the street so far at that pressure.
 

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I just put a set of 31/10.5R15 BFG KO2s on my 2000 xterra. The shop recommended a street pressure of 40lbs or so for those tires which is considerably higher than the stock pressure. My question is what should I air those down to for off-road use?

Oh by the way, a lot of people have reported running 31s on a stock 1st gen with no clearance issues but that was not my experience with these. Maybe it's just the way the tread wraps onto the side on these KO2s but I had to ditch the mudguard and trim off that lip because they were rubbing pretty bad.
The plastic on 2000 -2001 does stick into the well more, so, yes, you ditch some more plastic. The metal on the 2000 - 2004 are the same, but the plastic is different.


The tire pressure will depend on the tire and your rig's load.

On the sidewall of the tire, it lists the max load in pounds, and the max pressure in PSI.

What does it say on your sidewalls? (The sidewall doesn't say what #'s to use, it tells ME how to calculate it for you, with those #'s)
 

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Discussion Starter #5
[
The plastic on 2000 -2001 does stick into the well more, so, yes, you ditch some more plastic. The metal on the 2000 - 2004 are the same, but the plastic is different.


The tire pressure will depend on the tire and your rig's load.

On the sidewall of the tire, it lists the max load in pounds, and the max pressure in PSI.

What does it say on your sidewalls? (The sidewall doesn't say what #'s to use, it tells ME how to calculate it for you, with those #'s)
It's 2270 at 50lbs I believe. And my x is otherwise stock so the curb weight is right around 4000 lbs I think?
 

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Discussion Starter #6
Now that the work day is over I did a little research. If I'm understanding correctly how to calculate this then somewhere around 30 psi is right for the street. I'll be towing a travel trailer sometimes as well so I might keep it a tad higher than that. Thanks for the input guys.
 

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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.

:D


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.

:D
 

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That's a lot of great info! Thanks! Good thing I hadn't aired them down to yet! I was planning to though. I should have trusted my original instinct which was that 40 felt pretty good! I do have a couple of questions if you don't mind. What is that 5400 lb weight and what does it include? Is that a maximum? Or is it a realistic running weight? And should you run your tires for the max or for the weight you typically run? I'm just wondering what would be adding more than 1400 lbs to the curb weight? I thought curb weight included all fluids (gas, oil, etc.) which would mean I just need to account for passengers and cargo. My wife and I and our two kids are 500 lbs give or take (including car seats), and throw in maybe 300 lbs of gear for a camping trip and we are still at only around 4800 lbs total which would put the psi at around 37 if I'm doing the math right. 1400 lbs sounds more like 5 full grown adults plus gear!
I don't mean to sound like I'm doubting you. You obviously know a lot more about it than I do! I'm just trying to understand. The big thing that's confusing me a bit is how this all relates to the BFGs. Everything that I read about them was great and made them sound like they not only have very good traction but are very tough as well, but you described them as marginally too low on the load range. So my question is this - If the 31" ko2s are only marginally tough enough to handle the weight of a 1st gen xterra (which is a relatively quite small and lightweight suv), then what the heck are those tires designed for? They certainly wouldn't fit on my Yaris! :-D Though that would be kind of awesome if I could mount them on my Yaris... Also, what does that mean for the stock tires which had a load max of 2125?
Again, I'm really not trying to argue! This is just all new info for me and I like to really understand as much as I can when I get into something.
 

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Stock tires are to keep you from damaging the rims as you drive off the dealer's lot.

Some were very marginal.

Did you buy the 2000 X new?


The GVWR is on the placard on the driver's door jamb.

Its the max weight you'd run at....fully loaded.


The curb weight is typically empty of people and stuff, and with a tank of gas, oil and lubes, etc.

So, the difference between the curb weight and the GVWR is typically your maximum passenger and cargo weight.

You use the GWVR and not some other weight for the tire calculations because it gives a minimum fudge factor...and you don't have to go buy new tires because your kids get older and heavier or you go camping and then carry more cargo than when you bought the tires, etc.

:D
 

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I bought it new but they gave me a hefty discount because it took 15 yrs and 150,000 miles to get it to me! :D
I did a little research on what tires came on it originally and found the specs. Thanks again for all the info! This is all new to me (first 4x4) and I appreciate the help. I never had to think that much about the tires on any of my previous cars. Just replaced with same size as stock and kept the original tire pressure!
 

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You use GVWR, not curb weight.


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.

:D


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.
Hate to resurrect an old thread, but I've had a question about airing down. If you need x amount of PSI to support the load of the vehicle on road, how does the tire handle the same load off road with half the pressure? Just seems like there would be way more problems than normally occur off road. Especially when you slip off a rock or log and all that weight bounces down on the tire. Also, can you air down more with a higher load range? Since I have to be at 40-45PSI with these on road, D range might be 35PSI on road.

Wish I had re read all this tire stuff before I bought tires over the weekend. But now it doesn't appear they make the duratrac's in 31x10.5-15 with a load range D, so that's a little unfortunate (I know 32's fit with stock tires and are available with a higher rating, but mine with 31's are the most without rubbing).
 
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