Yet another tire pressure question.

[BathiasRei]

Howdy!

Currently dialing in my TJ's tire pressure to help on road manners and the consensus seems to have it settled at 28 PSI for E rated 235/75R15 BFGs. I've driven around town am enjoying the difference at sub 55 mph speeds but I'm curious if there's any draw-backs or cons that come with riding at a lower tire pressure than what is recommended in the door jam, especially at highway speeds. I daily the TJ around town with occasional highway driving and do some light off-roading on the weekends; since airing down on the street I feel that my braking distance may have worsened but I'm not sure if this is just placebo?

Thanks in advance and I apologize if this is common sense I'm missing!

 

[Woodrow]

Is that the stock size tire for your jeep? If so, the door jamb tire pressure is generally good. The confounding factor is the E load rating. Surely E rated tires didn’t come on the TJ new? I would guess C. If you are running a stiffer sidewall than stock, then airing down a few PSI makes sense. I doubt that would make your braking worse. It should increase your contact patch so I would expect better grip for braking

 

[BathiasRei]

Is that the stock size tire for your jeep? If so, the door jamb tire pressure is generally good. The confounding factor is the E load rating. Surely E rated tires didn’t come on the TJ new? I would guess C. If you are running a stiffer sidewall than stock, then airing down a few PSI makes sense. I doubt that would make your braking worse. It should increase your contact patch so I would expect better grip for braking

I believe the stock tire size for a 97 sport is 215 or 225 /75r15 and the door jam psi is 33/32 if I’m not mistaken? The tires I have currently came on the jeep when it was handed down to me… Mainly I just want to confirm the safety of airing them down a bit before I go into a long road trip w my gf. Thinking the braking is placebo or a difference from wearing boots instead of sneakers.

 

[Woodrow]

I believe the stock tire size for a 97 sport is 215 or 225 /75r15 and the door jam psi is 33/32 if I’m not mistaken? The tires I have currently came on the jeep when it was handed down to me… Mainly I just want to confirm the safety of airing them down a bit before I go into on a long road trip w my gf. Thinking the braking is placebo or a difference from wearing boots instead of sneakers.

Then you are running a slightly larger volume than stock tire. That and the E rating are both reasons to air down some. I think 28 is a good number. One easy way to look at your contact patch is by driving into a dry garage when its raining. If you don’t see the whole width of the tread, your pressure may be too high.

 

[Harold&Cat]

28 psi should be fine . If it is handling properly and the tires are not getting hot after a good run on the highway .

 

[LONGJP2]

Howdy!

Currently dialing in my TJ's tire pressure to help on road manners and the consensus seems to have it settled at 28 PSI for E rated 235/75R15 BFGs. I've driven around town am enjoying the difference at sub 55 mph speeds but I'm curious if there's any draw-backs or cons that come with riding at a lower tire pressure than what is recommended in the door jam, especially at highway speeds. I daily the TJ around town with occasional highway driving and do some light off-roading on the weekends; since airing down on the street I feel that my braking distance may have worsened but I'm not sure if this is just placebo?

Thanks in advance and I apologize if this is common sense I'm missing!

Not that critical.
Pressure can change 5+ psi just from temperature swings and/or the sun shining on one side or the other.

 

[deadbeat son]

I believe the stock tire size for a 97 sport is 215 or 225 /75r15 and the door jam psi is 33/32 if I’m not mistaken? The tires I have currently came on the jeep when it was handed down to me… Mainly I just want to confirm the safety of airing them down a bit before I go into a long road trip w my gf. Thinking the braking is placebo or a difference from wearing boots instead of sneakers.

Which size is on your door jamb sticker? 215 or 225? P metric, or LT? And what is the listed pressure?

An LT tire requires more pressure to carry an equal load to a P rated tire of the same size. (Recall the Ford Explorer / Firestone debacle of the early aughts.)

 

[BathiasRei]

Which size is on your door jamb sticker? 215 or 225? P metric, or LT? And what is the listed pressure?

An LT tire requires more pressure to carry an equal load to a P rated tire of the same size. (Recall the Ford Explorer / Firestone debacle of the early aughts.)

I can't say off the top of my head but I can check tmrw and upload a picture

 

[deadbeat son]

I can't say off the top of my head but I can check tmrw and upload a picture

The placard on the driver's doorjamb of my old TJ said the stock tire size was P215/75R15, and the recommended inflation was 33psi (both front, and rear)

Looking at tire load inflation tables, that size tire is rated to support 1,664 lbs at 32psi and 1,742 lbs at 35psi. From these figures, you can reasonably estimate that each psi over 32psi supports an additional 26 lbs. Therefore, the factory tire size supports 1,690 lbs at 33psi when used in a passenger car application.

However, the TJ is not a passenger car application, so the tire's load capacity has to be de-rated by 10%. 1,690 * 90% = 1,521 lbs.

Moving to a LT235/75-15 in the load inflation chart, that size tire is rated to carry 1,530 lbs at 35psi and isn’t rated at pressures below that.

The 1,530 lb load rating is reasonably close enough to the OEM spec of 1,521 lbs that you should run 35 psi with an LT tire in the size you’re running to meet the factory specs for load capacity. Under inflation and highway use can cause excessive heat to build up in the tire and cause a blowout.