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#4632997 - 01/12/18 08:57 PM Re: What tire pressure to run? [Re: HoosierJeeper]
bmwtechguy Online   content


Registered: 06/15/04
Posts: 2678
Loc: South Carolina
I generally fill all tires to 38-40 psi due to the fact that most people never check their tire pressure unless a light comes on, and the fact that tires constantly lose a small amount of air at all times. I put 4 new tires on an older Civic for a friend that was on hard times a couple of years ago and just saw her car again after all this time. I knew I had put in 38-40 psi as usual when the tires were new. The car was back this time for worn-out front brake calipers (252,000 miles on originals. Now she has a job and has money to pay for the work.) Anyway, all of the tires had 18-19 psi in them. Surprisingly, they were not worn badly on the edges. I told her she needs to check her tire pressure and keep filled for the obvious reasons.

If you put in what the door sticker says, you are already low on air pressure, IMO.

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#4633036 - 01/12/18 09:30 PM Re: What tire pressure to run? [Re: bmwtechguy]
Dumc87 Offline


Registered: 06/05/10
Posts: 591
Loc: Florida
Originally Posted By: bmwtechguy
I generally fill all tires to 38-40 psi due to the fact that most people never check their tire pressure unless a light comes on, and the fact that tires constantly lose a small amount of air at all times. I put 4 new tires on an older Civic for a friend that was on hard times a couple of years ago and just saw her car again after all this time. I knew I had put in 38-40 psi as usual when the tires were new. The car was back this time for worn-out front brake calipers (252,000 miles on originals. Now she has a job and has money to pay for the work.) Anyway, all of the tires had 18-19 psi in them. Surprisingly, they were not worn badly on the edges. I told her she needs to check her tire pressure and keep filled for the obvious reasons.

If you put in what the door sticker says, you are already low on air pressure, IMO
.


I would have disagreed 4 months ago. Now, having been a tire guy for a few months, I 100% agree.
_________________________
'90 Z32 - PP HM 10w30
'98 ZJ - PYB HM 10w40
'02 G20 - QS UD 5w30
'09 Matrix - QS UD 0w20

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#4633093 - 01/12/18 11:14 PM Re: What tire pressure to run? [Re: HoosierJeeper]
zeng Offline


Registered: 09/01/15
Posts: 2199
Loc: Malaysia
Originally Posted By: HoosierJeeper
Feels like I've been posting a lot in this section lately... oh well, I'll try to stop eventually. smile

Walmart put 35PSI in my new tires (245/75/16 Hankook Dynapro ATMs, P rated). Does that sound about right for my Liberty? Weight wise I'm guessing it's probably around 4600-4800 lbs once you add in the winch, skid plates and rock rails. Door sticker is irrelevant at this point, I think, not a factory size. I'd do the chalk test but I'd have better luck playing pickup sticks with my butt cheeks than finding a dry section of pavement here for awhile. smile

Walmart did right with 35 psi. No worry.

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#4633138 - 01/13/18 12:15 AM Re: What tire pressure to run? [Re: HoosierJeeper]
BrocLuno Offline


Registered: 09/06/15
Posts: 5613
Loc: Kalifornia Kollective
This is a 4 wheel drive, so conditions indicate what the pressure should be ... Sand, maybe 15 psi. Rocky trail, maybe 36 PSI. Desert trials, maybe 28 PSI. Highway, whatever gives you even tire wear across the tread area.

BUT, 4x4's are different from FWD or RWD in that they bind if the rolling diameter is off. So you not only carry a tire pressure gauge, but you carry a tape measure. You air up the fronts to whatever the driving conditions are - let's say highway at 36 PSI.

On a clean dry level surface like a parking lot, you measure from the ground to the upper lip on the front wheel rim. Say it's 20". Now you inflate the rears so you get the same 20". Pressure makes no mind. You must be at rolling the same dia ...

With nothing in the back - it could be 26 PSI. Or with the back full of camping gear, it could be 34 PSI. As long as it's the same rolling dia, it'll shift into and out of 4x4 on the fly with minimal bind. Get them off and you'll have to stop and back up to get it to unlock ...

On 4x4's you must learn to manage the pressures for the conditions at hand, and there will be a difference between front and rear because of vehicle loading, engine weight, etc. smile


Edited by BrocLuno (01/13/18 12:19 AM)
_________________________
Formerly in marine engineering. In an earlier life I owned my own petroleum tank truck, and hauled for the majors and independent's.

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#4633171 - 01/13/18 02:47 AM Re: What tire pressure to run? [Re: HoosierJeeper]
Shannow Online   content


Registered: 12/12/02
Posts: 40156
Loc: 'Stralia
Broc, I disagree...the tiniest difference will lead to wind-up, as does the Ackerman effect on the steering, which means that all the tyres run a different number of miles in any single trip.

Only technique that I've found for all the variables is to take an engineer's 6" rule, and slip it in between the tread and the concrete as you pump the tyres.

about a half inch of peneetration seems to give fairly even wear...too much penetration, and there's too much pressure. Too little, and there's not enough. As you drop the pressure, you can see how hard the edge is to lift, as the centre of the tread "pops" upward with no contact pressure really at all.

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#4633332 - 01/13/18 08:37 AM Re: What tire pressure to run? [Re: HoosierJeeper]
CapriRacer Offline


Registered: 02/22/03
Posts: 3385
Loc: Somewhere in the US
Here's how to do this:

The principle is to match the original load carrying capacity. The starting point is the vehicle tire placard.

I can't tell which jeep your new tires went on, so I am going to guess the '05. Tire Guides doesn't list the various Jeep models as you've described them in your signature, but I am going to take a guess the pressures listed on the vehicle tire placard are 33/33 and the tire size is P225/75R16. Please check to be sure that is correct.

If that is correct, in order to carry the same load as a P225/75R16 does at 33 psi, a P245/75R16 needs to use 26 psi.

- BUT -

If you can match the load carrying capacity at a lower pressure, it's a good idea to match the original spring rate of the tire, which is mostly about inflation pressure (unless you are changing springs and shocks, too!) - so I would recommend using the originally spec'd 33 psi.

And that would be your new starting point. Some folks like to modify from there to achieve a different goal that the engineers at the factory had in mind.

Oh, and one other point: A P245/75R16 requires a minimum of a 6 1/2" wide rim. I think your Jeep has 7" wide wheels, but you need to check. If you try to use a rim that is too narrow, the vehicle will wander and follow the ruts in the highway, as well as be sensitive to cross winds.
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CapriRacer

Visit my web site: www.BarrysTireTech.com

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#4633418 - 01/13/18 09:36 AM Re: What tire pressure to run? [Re: HoosierJeeper]
HoosierJeeper Offline


Registered: 11/23/16
Posts: 1276
Loc: WI
Thanks for the help everyone!
@Capriracer, I'm running 8" wide rims now, how does that effect pressure? (vs 7 stock)
_________________________
15 Jeep KL Limited V6: Formula Shell 5W20
07 LR3 SE V8: Maxlife 5W30
05 Jeep KJ Limited: PHM 5W30
96 Jeep XJ Country: STP HM 5W30 (winter)

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#4633650 - 01/13/18 01:21 PM Re: What tire pressure to run? [Re: HoosierJeeper]
CapriRacer Offline


Registered: 02/22/03
Posts: 3385
Loc: Somewhere in the US
Originally Posted By: HoosierJeeper
Thanks for the help everyone!
@Capriracer, I'm running 8" wide rims now, how does that effect pressure? (vs 7 stock)


From a load carrying capacity point of view, it doesn't.
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CapriRacer

Visit my web site: www.BarrysTireTech.com

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#4638383 - 01/18/18 02:17 AM Re: What tire pressure to run? [Re: HoosierJeeper]
carloz Offline


Registered: 01/03/17
Posts: 44
Loc: Fl
30-32 is the ideal pressure, though 35psi won't cause serious problem.

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