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Thread: ZJ Tire Pressure Question

  1. #1
    Registered jbizub5192's Avatar
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    Default ZJ Tire Pressure Question

    Hey everyone!! I always used to run my tires 2 psi above the 36 what the factory setting is. I have a 2in bb on with all new shocks and a steering stabilizer and my tires are 30.5s. What pressure do you suggest I should have in there? The only reason i had more pressure was for better mpg for the niner but it might not make that much of a difference with a little less lol.

    Thanks
    1998 Grand Cherokee Limited 5.9L

  2. #2

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    Lift height, shocks, SS, etc, does not matter. You need to base it off of the tire manufacturer's specs.

    I had 31" BFG A/T's on the Jeep, the max was 50 PSI, I ran them at 40-45 PSI. Wear pattern and ride quality were fine.

    Right now my ZJ is on 33" Duratracs, the max is 35 PSI and I'm running 25 PSI. As above, ride quality is great, and it is daily driven in the city and on the highways frequently.


    My recommendation would be to start at 5-7% less than the maximum listed PSI on the tire. See how it feels and wears, and go from there. Run whatever PSI you like best, as long as the wear pattern is constant and even. I would not run them over the maximum PSI.

  3. #3
    Registered louc's Avatar
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    When it come to tire pressures, you have to be careful, in that LT rated tires of the same size as P rated tires need more air pressure to carry the same load. I researched this quite a bit and it's not well understood, but you are starting with a ZJ that originally came with P-rated 215/75-15 tires, or on the 5.9, 225/70-16s, and those 215s were rated at max pressure at about 1764 lbs, (maybe a bit more for the 5.9...1860 or something close) at their max pressure, but when you use P rated tires on a truck they have to be de-rated by 9%. That gives you 1605 per tire, that's what jeep felt the minimum weight capacity for a ZJ at full load should be per tire. Then you need to make sure that whatever tire you put on, can support at least that much weight if not more, at the inflation pressure you are using. I did the #s on my ZJ, if I used a LT 235/75-15 in place of the OE 215s, I have to go to 38-40 psi for adequate load capacity. The issue here is that they build in a large safety margin, obviously a stock ZJ weighs in at about 4000 lbs, that's 1000 lbs per tire, but that's only static weight distribution, not dynamic were the load on each tire can be a lot more, as the weight shifts under acceleration, braking etc.

    Goodyear published a load inflation chart, on their website in the RV section, that tells you what various size LT tires can support weight wise at different pressures. You can use this as a guide as to how much air pressure is needed...

    http://www.goodyear.com/rv/pdf/rv_inflation.pdf


    The main issue is having enough air pressure, that the tire does not overheat at highway speeds in hot weather, both Nitto Tires and Yokohama have stuff about this on their website....
    Lou C

    98 ZJ Laredo 4.0 Selectrac
    H-4/ECE Headlights, Painless Wiring Harness, Hella 450 fogs
    Front and rear tow hitches, Air Lift, Bilsteins
    Dynomax Cat Back Exhaust
    B&M Supercooler

    07 WK Limited 5.7 HEMI Quadradrive II


    1988 Four Winns 200 Horizon
    4.3 OMC/Cobra Drive

  4. #4

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    Factory inflation on my 94 ZJ door sticker is 33 PSI, not 36PSI.
    94 5.2 Ltd. Selec Trac
    00 4.7 Ltd. Selec-Trac
    98 5.9 Ltd.
    95 5.2 Ltd.
    62 CJ-5 - Chev 283 V8 - OD

  5. #5

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    louc

    I just put on a new set of Michelin LTX M/S Highway all season tires in 215/75/15 (1998 Loredo 4.0).

    Should I stay with the 36 psi it says on the door jamb? It's an SL tire.

    Thanks.

  6. #6
    Registered louc's Avatar
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    Quote Originally Posted by duckstu View Post
    louc

    I just put on a new set of Michelin LTX M/S Highway all season tires in 215/75/15 (1998 Loredo 4.0).

    Should I stay with the 36 psi it says on the door jamb? It's an SL tire.

    Thanks.
    That should be fine, as long as it rides the way you want, the 36 psi is for full load conditions (says in the glove compartment sticker), if you feel it's too stiff you might be able to lower it a bit, but I usually err on the side of a little more pressure because the tires run cooler then....
    Lou C

    98 ZJ Laredo 4.0 Selectrac
    H-4/ECE Headlights, Painless Wiring Harness, Hella 450 fogs
    Front and rear tow hitches, Air Lift, Bilsteins
    Dynomax Cat Back Exhaust
    B&M Supercooler

    07 WK Limited 5.7 HEMI Quadradrive II


    1988 Four Winns 200 Horizon
    4.3 OMC/Cobra Drive

  7. #7
    Victim of Circumstance HighDesertZJ's Avatar
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    Default

    Quote Originally Posted by Tcar45 View Post
    Factory inflation on my 94 ZJ door sticker is 33 PSI, not 36PSI.
    Yep, I run 33 psi.

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    Registered jbizub5192's Avatar
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    Thanks everyone. hmm thats weird. do you guys have a v8 or v6? My defiantly says 36 for the factory psi. Right now for the past 2 days i lowered mine to 35 psi from 38 and it was like a night and day difference as far as the ride. It rides so much smoother. I forgot to say before my tires are 245/75/16 Bridgestone Dueler A/T Revo 2
    1998 Grand Cherokee Limited 5.9L

  9. #9

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    Mines a 94 Limited V8. 33 psi on the door. There is no allowance for max load.

    The 93-95 had 15" wheels, don't know if that makes a difference. Don't know why it would.

    I now have 16x8 - Moabs from a Rubicon Wrangler - on it now. Maybe I should try 36# instead of 33.
    Last edited by Tcar45; 03-29-2011 at 12:23 PM.
    94 5.2 Ltd. Selec Trac
    00 4.7 Ltd. Selec-Trac
    98 5.9 Ltd.
    95 5.2 Ltd.
    62 CJ-5 - Chev 283 V8 - OD

  10. #10
    Registered jbizub5192's Avatar
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    Default

    Maybe thats the reason then, just a different rim size. Im sure someone on here could clarify that lol
    1998 Grand Cherokee Limited 5.9L

  11. #11

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    Different size tires and the intended vehicles (P vs. LT) will require different pressures. Although usually not huge differences they will be different. The air is actually what supports the vehicle not the tires, the tires are merely the capsule for the air. The ideal way is to weigh the vehicle at each corner and then match to the tire manufacturers weight/tire pressure chart. Which ever corner is the heaviest use that rating to find the recommended tire pressure. On a 4wd vehicle make certain the tire pressures and sizes are all the same.

    Another way is to park on a level surface with enough length to drive a few hundred yards straight without turning the steering wheel. A large parking lot works great for this. Take a piece of chalk and color a section of the tread about three to four inches wide across the entire tread width. Drive straight ahead a hundred yards or so without turning the steering wheel. Look at the chalk line, it should all be completely gone clear outer edge.

    If it has remaining chalk in the middle it is underinflated, chalk on the outer edges it is overinflated. This is not as accurate but seems to work well in a pinch.

    P.S. On my 96 ZJ with 31x10.5x15 BFG All/Terrains I am sitting right at 36 psi for weight and best ride quality. Mike.

  12. #12

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    The chalk test is a simple way to get the pressures set for any size tire or load. If done correctly, it'll give you the correct psi every time.
    1997 Limited 5.2, Atlas TC, 7" Claytons LA, 488 gears, hp30 w/oxlocker, 8.8 w/ARB, Alloys F&R, Ballistic Crossover steering, bumpers & winch, ORO dual rate swaybar, 35" KM2s, hydro steering assist.

    1997 TJ 350, 700R4, D60, 14 bolt, Atlas, Full Hydro steering, 42" Pitbulls, Stazworks beadlocks, Locked F&R, 14" ORI Struts, more

  13. #13

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    Make sure, though, if you have a quadra- anything transfer case, you need absolutely equal tire pressures all around.
    94 5.2 Ltd. Selec Trac
    00 4.7 Ltd. Selec-Trac
    98 5.9 Ltd.
    95 5.2 Ltd.
    62 CJ-5 - Chev 283 V8 - OD

  14. #14
    Registered louc's Avatar
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    True you want the tire circumference to be the same as close as possible...

    and about rim size, well for 6 cyl ZJs I think they all came with a 15x7 rim, some of the V-8s came with 16", not sure of the width...but they may have increased the psi to add to the safety margin, because the curb weights of the ZJ (3800-4000 depending on equipment) did not change that much...


    Back to my points....first I figured out what Jeep felt the mininum weight capacity was at full load (using the base 215-75-15 tires, de rated 9% because they are P rated on a light truck). That gives you 1605 lbs, so whatever tire you put on at whatever inflation you use, it should not be less than that. They obviously built in a safety margin, that's why ZJs did not have the blow out issues Ford Explorers had, they use very low inflation pressures of 26 psi, vs ZJ at 36. If you underinflate LT tires they will run hot and can fail, this is important and I would not leave it only to the chalk test, you need to know how many pounds a given tire will support at a certain pressure if you want to keep the same safety margin...
    Look at the chart I linked above....
    Last edited by louc; 03-30-2011 at 05:23 PM.
    Lou C

    98 ZJ Laredo 4.0 Selectrac
    H-4/ECE Headlights, Painless Wiring Harness, Hella 450 fogs
    Front and rear tow hitches, Air Lift, Bilsteins
    Dynomax Cat Back Exhaust
    B&M Supercooler

    07 WK Limited 5.7 HEMI Quadradrive II


    1988 Four Winns 200 Horizon
    4.3 OMC/Cobra Drive

  15. #15

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    Quote Originally Posted by louc View Post
    6 cyl ZJs I think they all came with a 15x7 rim, some of the V-8s came with 16", not sure of the width...but they may have increased the psi to add to the safety margin, because the curb weights of the ZJ (3800-4000 depending on equipment) did not change that much.
    Not quite.

    All ZJ-1 (93-95) had 15" wheels.

    Some ZJ-2 (96-98) had 16" wheels. All 5.9's had 16".

    I don't see how tire diameter should affect the psi recommended.
    94 5.2 Ltd. Selec Trac
    00 4.7 Ltd. Selec-Trac
    98 5.9 Ltd.
    95 5.2 Ltd.
    62 CJ-5 - Chev 283 V8 - OD

  16. #16
    Registered jbizub5192's Avatar
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    Thanks everyone for all the info! I will try the chalk and checking the weight in the upcoming weeks. For now I have the tires at 35 and it rides so much smother the 38 lol


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    1998 Grand Cherokee Limited 5.9L

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