Tyre Pressure Query

jpjeep26uk

jpjeep26uk

YJ Enthusiast
Joined
Nov 12, 2022
Messages
198
Location
UK
Hi all, I'm running BFG LT215 75 R15 K02's on my early 1989 YJ. It's a Sahara and I don't off road it. It's got these tyres on as it's a Jurassic Jeep Replica. I just run about on the roads to events and stuff. What should my tyre pressures be? My jeep weighs about 3085lbs with me in it and all the jurassic add-ons ( lightbar on the roof). Currently at 26psi fronts and 28psi rears. Thanks all.

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Factory tire pressure for P215/75R15 SL was 28F/32R.

At 28 psi, that size tire can support 1561 lbs. At 32 psi, they can support 1664 lbs.

With an LT tire, even at the same size the load is drastically changed. LT Metrics aren't even rated below 35 psi. Also, with passenger load tires, you add in 10% overage in load capability because they're weaker tires. So if you needed the stock tire to support 1500 lbs, you would inflate it to a pressure that supports 1650 lbs. When running LTs, you remove that overage factor.

So, you have to take out that overage for your LTs, meaning you need an LT tire to support 1419 lbs up front and 1513 lbs in the rear instead of 1561 & 1664 lbs.

Next, you have to look at what weight an LT C tire in the size you have can support at what psi. LT215/75R15 C is rated to support 1397 lbs at 37 psi and 1423 lbs at 38 psi. So, technically, you should be running 38 psi in your size up front to replace the stock P215/75R15 SLs (remember, you're trying to match the 1419 lbs required which came from adjusting the stock pressure and reducing it by 10%).

Same deal on the rears, at 41 psi, LT215/75R15 is rated to support 1501 lbs, and at 42 psi it's rated to support 1527 lbs. You want 1513 lbs so you need to go with 42 psi..

So technically, you should be at 38F/42R.

You might find that a bit harsh in ride quality, due to the stiffer LT tire, but those are the pressures where they will have the same load capacity as the stock tires at the stock pressures, and should theoretically wear the same. Of course, you may be able to get away with less pressure and no ill effects, but no telling for sure on that.

The owner's manual also says to bump the pressure up 3 psi if you'll be driving at more than 74 mph continuously for a while, like a road trip...in that case, you'd want 41F/45R psi, based on the stock pressures being upped from 28F/32R to 31F/35R.

You could play around with it some and see what you like but my guess is currently you're significantly underinflated. Sometimes though, it really doesn't matter so it may just be fine.

This tire pressure calculator website is a really handy way to calculate what you need based on the OEM recommendation. The trick is using it right. If your stock tires had a P in front, you need to choose P metric. If they were just the numbers, you choose euro metric. If they were LT metric, you choose LT metric, and if they were high flotation (31x, 33x, etc), you choose "LT".
 
Low tire pressure causes excessive heat buildup leading to tire failure. The higher the speed and heavier the load will increase heat buildup.
 
Sheeeeetttt....Ive been running my 91's tires at about 18 for 4 years now. That includes the occasional trip to town which is 120 miles. You just need some good Taiwanese tires like I have :cool:
 
Gilaguy23 said:
Sheeeeetttt....Ive been running my 91's tires at about 18 for 4 years now. That includes the occasional trip to town which is 120 miles. You just need some good Taiwanese tires like I have :cool:
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You have LT 31x10.50s though don't you? Huge difference between those and LT metrics, especially the smaller ones in the original post. For example, I would start their pressure calculation off of the stock pressure of P205/75R15 for a 4 cylinder, and those tires themselves are both larger and different strength than the ones in the OP. Your calculated pressure would be more like 25 psi, which is the lowest a "31x, 33x, 35x" etc is supposed to go. But 18 is not that far from 25 and you're running a 4 cyl so it works.
 
OK, I threw in the required amount of air and yeah, she sita about an inch taller haha, rides like like a brick on rocks but for sure she rolls much more freely and doesn't feel like I'm driving though toffee or taffy as its called in the States I think. Either way thanks folksnand I'll see how she fairs this weekend with some long distance coming up.
 
jpjeep26uk said:
OK, I threw in the required amount of air and yeah, she sita about an inch taller haha, rides like like a brick on rocks but for sure she rolls much more freely and doesn't feel like I'm driving though toffee or taffy as its called in the States I think. Either way thanks folksnand I'll see how she fairs this weekend with some long distance coming up.
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I was thinking you had a 4.2L since Sahara....basing the pressure off of the stock 2.5L tires of P205/75R15 28F/32R, you could actually run 35 psi front and 37 psi rear on the LT215s.
 
machoheadgames said:
You have LT 31x10.50s though don't you? Huge difference between those and LT metrics, especially the smaller ones in the original post. For example, I would start their pressure calculation off of the stock pressure of P205/75R15 for a 4 cylinder, and those tires themselves are both larger and different strength than the ones in the OP. Your calculated pressure would be more like 25 psi, which is the lowest a "31x, 33x, 35x" etc is supposed to go. But 18 is not that far from 25 and you're running a 4 cyl so it works.
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Yeppers. I typically use mt YJ locally on forest roads and am honestly to lazy to reinflate them. Gotta say these Taiwanese made tires wear like iron, least the ones Ive bought. Kinda wish they would die so I could get some muds.
 
Had my Kirkland tires for 15yrs. Side walls finally gave out and still had 80% tread left. I only put about 500mi/yr mainly off road.
 
machoheadgames said:
I was thinking you had a 4.2L since Sahara....basing the pressure off of the stock 2.5L tires of P205/75R15 28F/32R, you could actually run 35 psi front and 37 psi rear on the LT215s.
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OK I'll try that too, see what's what and come back to report.
 
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