Is 8 psi in tires OK at highway speeds???

OK, I got your attention.

Facts: Airing down in the dirt definitely helps. 8 psi on pavement leads to Phoenix-Firestone-blowout-rollover headlines. 96XJ, manual lists at 3000#. Manual says 33-35 psi. 235/75-15 Goodyear Wrangler MT/R. Sidewalls say 1985# capacity each @ 50 psi. Stock rims. SWAG on loaded weight 4000#.

Questions: What pressure works in the dirt? Rocks? Mud? Snow? Front and rear the same? How far do I need to go back up for 60 mph in 90 degree temps?

I know there must be a way to calc it, but just guessing that a percentage of sidewall rating pressure correlates to a percetnage of tire rating spooks me.

Man is that a loaded question. What size tire are we talking about? what size rim? I remember someplace seeing someplace on the net a chart that had some info as to what others use happily on the trail. 8psi sounds on the low side and I can see someone easily loosing a bead at that pressure. I run 31's at anywhere from 12 to 15.

Anyways in regards to driving on the freeway, when I leave the trail I carry a truck compressor (~$50 at Kragen) which works semi ok to get me to 30psi. If I drive at lower pressure (multiple trail runs from same area and just trail access) I drive slowly on the freeway as sudden turns/maneuvers can pull the tire of the rim.

PlainWhiteXJ said:

OK, I got your attention.

Facts: Airing down in the dirt definitely helps. 8 psi on pavement leads to Phoenix-Firestone-blowout-rollover headlines. 96XJ, manual lists at 3000#. Manual says 33-35 psi. 235/75-15 Goodyear Wrangler MT/R. Sidewalls say 1985# capacity each @ 50 psi. Stock rims. SWAG on loaded weight 4000#.

Questions: What pressure works in the dirt? Rocks? Mud? Snow? Front and rear the same? How far do I need to go back up for 60 mph in 90 degree temps?

I know there must be a way to calc it, but just guessing that a percentage of sidewall rating pressure correlates to a percetnage of tire rating spooks me.

Click to expand...

Oops I just reread your question ANyways stock rims with 235's I ran at no less then 14psi and I made sure to get close to being fully aired up before reaching highway speeds.

There was an article years ago that I read in one of the 4wd rags that was written by an engineer for Interco and in it he stated to do exactly what you are leary of..... divide the actual load rate by the capacity then multiply the max pressure by what you got from that.
IE - 1000/1985=.504x50=25.2
Now that was Interco and it has worked well for me, on all the TSL's I've run I always ran about 25 psi, but they have pretty stiff sidewalls, I don't think the MTR's are quite as stiff so that might not apply.
The old skool method is to find a nice flat piece of pavement, take a piece of colored chalk and draw a line across the tread surface, then drive 20-30 ft. If the chalk wore off all the way across then your on the money if it leaves some chalk on the outside air down a few psi, if some is in the middle air up. This method will get you pretty close and allow you to figure if you should run more or less in the front & rear, I suppose, to compensate for any extra junk you might have hanging off the Jeep like tire carriers, winches ect....
HTH,
Mike B.

i think i saw it at gojeeps site, he said the proper air pressure for road use is to check the tire pressure cold, then drive 20 minutes, you should have a 4 psi increase in pressure, more than that then you had to little to begin with, less and there was to much in the first place...

http://www.go.jeep-xj.info/HowtoTyrePSI.htm

I typically air down a lot less than folks I wheel with, because I don't have a good way to air up again when I'm done for the day. So I won't even address what works off-road. A lot depends on the particular tire, too, as well as the terrain.

Don't forget that at 90 degrees air temperature, if the sun is out the pavement temperature will be a LOT hotter. The figures you cited from the Jeep manual are, I believe, the MAXIMUM pressures for towing or hauling full loads. I run my XJs (on 225/75 or 235/75 tires) at 30 psi and I get good fuel and tire mileage. I have run as low as 26 psi coming home from a day of wheeling, but that was because I ran out of quarters at one of those robber stations that charges for air. I typically regard 28 psi as the minimum safe pressure for highway travel.

Larger tires need less pressure to carry the same load and keep the tread flat, but what builds up the heat is sidewall flex. Bias ply tires are more susceptible to this than radials. So regardless of tire size, you need enough pressure to keep the tires from overworking. The 4 psi rise test is a decent way to get some idea of if you're too low, but I don't think it's 100% reliable. I'd consider 26 to 28 psi an absolute minimum and go from there. Many years ago, in my pony car days, we routinely ran radials at 24 psi, but the radials in those days were engineered to be run at lower pressures.

I believe Eagle may have omitted the P from his tire size? That makes quite a difference as P series tire typically have much softer sidewalls than LT's. In the case of P series tires I completely agree with what he said... In the case of LT's I stand by what I wrote earlier, the 36" Bias TSL's on my CJ show no sidewall flex at all till well under 20 PSI, and I run them at 8 psi on the trails.
Mike B

Here's what happens to a GY MT (bias ply) with low pressure at freeway speeds on a 100 degree day. :wave:

8 is way to low for highway speeds. I seldom air my 31's to eight. Mostly stick to 10-12 but i have a compressor and air them back to 30.

If you have no way to air up then stop at 20 and drive slower, like under 50. If you want to drive faster then don't air down unless you are stuck.

xjhm said:

i think i saw it at gojeeps site, he said the proper air pressure for road use is to check the tire pressure cold, then drive 20 minutes, you should have a 4 psi increase in pressure, more than that then you had to little to begin with, less and there was to much in the first place...

http://www.go.jeep-xj.info/HowtoTyrePSI.htm

Click to expand...

The rate of expansion will vary by quite a bit with changes in the moisture content of the air. Dry air will expand less that moist air. This method would only work if you adjust for humidity.

MaXJohnson said:

The rate of expansion will vary by quite a bit with changes in the moisture content of the air. Dry air will expand less that moist air. This method would only work if you adjust for humidity.

Click to expand...

I live in Northern New Mexico. We don't have humidity!

MaXJohnson said:

The rate of expansion will vary by quite a bit with changes in the moisture content of the air. Dry air will expand less that moist air. This method would only work if you adjust for humidity.

Click to expand...

Yep and and still doubt it will amke much of a difference with moist air. I've checked my tire from when I filed them at a cooler temp(65) then ran 120 miles as the day warmed up to about 80 and checked them when filling for gas and saw maybe 1 psi difference. Rapid City, Sd to Gillette, WY.

I just came across this and remembered your post: http://californiajeeper.com/tire-presures.htm

PlainWhiteXJ said:

OK, I got your attention.

Facts: Airing down in the dirt definitely helps. 8 psi on pavement leads to Phoenix-Firestone-blowout-rollover headlines. 96XJ, manual lists at 3000#. Manual says 33-35 psi. 235/75-15 Goodyear Wrangler MT/R. Sidewalls say 1985# capacity each @ 50 psi. Stock rims. SWAG on loaded weight 4000#.

Questions: What pressure works in the dirt? Rocks? Mud? Snow? Front and rear the same? How far do I need to go back up for 60 mph in 90 degree temps?

I know there must be a way to calc it, but just guessing that a percentage of sidewall rating pressure correlates to a percetnage of tire rating spooks me.

Click to expand...

Thanks Katjar, that's the most detailed I have seen.