How low can I go?

[vseasport]

Looking for opinions from modified Jeep owners. Recently purchased a 2015 Wangler Willys. It is lifted 2 1/2" and has new 35" wheels and tires. The door frame sticker indicates the stock tire should have 37psi. The manufacture of the 35's says run 26psi. So, I have little experience off road. If we were to plan a 75 or 100 mile trip on a dirt washboard road should the tire pressure remain at 26psi? How low can I go without damaging the tires and improving the ride.

[Chargerman]

You can air down to what the tire manufacturer recommends. If you are just driving on gravel or hardpacked dirt roads I would just leave the pressure at what is recommended and normal driving on the door sticker.

[Crasher]

When I had a Jeep on 35" tires, I carried 12-14 psi when off road. That gave a good ride and improved traction. Rarely went over 35 mph. If you will be traveling 45-50 mph on a rough dirt road, I would run 18 psi. A safe way to calculate off road pressures is to weigh each position. Then look at a load inflation chart for your tire with the weight you have. That would give you a safe pressure for the highway. 65- 75% of that pressure would work well off road.

Edit: What harms underinflated tires is speed and heat. The 35" tires I had required 22 psi by weight for highway travel and worn even for three years.

[vsheetz]

A lot depends on the specific tires and wheels. I've a Jeep specifically setup fot difficult off-roading and rock crawling and routinely air down to 10 psi. But thats not for you and what sounds like an over landing type travel.

Another consideration is how are you going to air the tires back up once back on the pavement and going highway speeds. I have an onboard air compressor setup. Portable compressors and CO2 tanks are options.

A cursory Google found this which I think is a good summary.

25%
As a (very) rough guide, dropping pressure about 25 percent*will significantly increase traction and comfort on rough dirt*roads, washboards, and moderate trails.

30-35%
Dropping 30 to 35 percent elongates the tire’s footprint*enough to enhance grip on slickrock, and allows it to deform*around and cling to big rocks.

50%
Heading down to 50 percent, even a bit less, will nearly double*the length of the contact patch to float over very soft sand.

[NevadaNick]

When i had my Bronco i ran 15 psi and even drove it on the hwy for 50 or so mi at the speed limit

[vseasport]

Thanks for the replies.

I have Nitto Trail Grapplers 35X12.50 R17.

A search brought up a bunch of hair ball info but not a chart to help figure it out. Factory engineer said run 26PSI which will be a good starting point for highway. For day trips from our RV on bad roads I'll lower them down 25%. Just don't want to get stranded 100 miles away or mess up the tires. Air up will be no problem once back at camp.

[153stars]

If your going to be 4 wheeling way out on your own, you should have a way to air up and repair a leak besides a spare and a stock jack.

[Crasher]

I find it hard to believe that 26# is the right pressure for highway driving with your Jeep on 35" tires. I would bet that will give a rougher ride and will wear out the center of the tread, because they are overinflated for the load they are carrying. Go weigh each corner and go to a load inflation chart for your 35" Nitto Trail Grappler. That's a great off road tire and gives excellent performance at 10-12 psi on a 4500# Jeep. I have friends that run 8-10 psi off road on their 40" Nitto's with great success. Don't just guess at it. Weigh it and get it safe and correct.

Edit: Each tire can carry 3640# at sidewall pressure. On your Jeep, they are carrying 1200 to 1250# or less. You only want enough pressure to give even contact across the tread to be road safe. When you go to a tire size different from the OEM size, the tire placard on the Jeep is meaningless.

[petrel]

My DW has the same make and size tires on her Jeep. However, they are Ridge Grapplers instead of Trail Grapplers. 33 psi is what she usually runs, unless airing down for some challenging off road adventure. This seems to work fine for fire roads, and highways. They appear to be wearing fine (or hardly at all), and we haven't noticed that it is riding particularly rough.

[Crasher]

Here is a chart for a Toyo MT 35X12.50/17 tire. A different brand, however, all brands with the same specs will have very similar pressures. Note the the correct pressure for a tire carrying 1875# is 25 psi. The OP's Jeep is probably about 1250# or less. According to the tire manufacturer, 25 psi is over inflated for his Jeep. My guess the correct pressure after he has confirmed the weight, will be about 20-22 psi. Every tire manufacturer will recommend the pressure required to carry the given weight. That pressure will give the best traction both for acceleration and braking plus the best wear pattern. The ONLY way to get the correct pressure for any tire is to weigh what it is carrying and set the recommended pressure for that amount. Most of us do that with our motorhome for the best performance. The same applies to any vehicle with inflated tires. I've always felt that the tire manufacturer knows more about tire performance and safety than me any vehicle manufacturer.

https://www.toyotires.com/media/3729...s_20200723.pdf

[twogypsies]

Originally Posted by vseasport

Just don't want to get stranded 100 miles away or mess up the tires. Air up will be no problem once back at camp.

Most 4x4 drivers carry a portable air compressor. We carried a Viair which is popular with them. Going out we aired down at the trail beginning and aired up as soon as we finished. We certainly didn't wait until we got back to camp.

[vseasport]

So, here is a little more background on my Jeep.

After the purchase I found it pulling to the left. The prior owner said it had just been aliened. I called Nitto (The factory not a dealer) and was told to take it to Americas Tire because they did warranty work for Nitto. They also went through a long calculation and said my specific Jeep should run 26psi and wrong psi could contribute to it pulling.

I paid Americas Tire $100 to do a "Road Force Balance". Before getting home they called and said pick up the car. The manager gave my money back and said there is nothing wrong with the tires. He didn't have the equipment for alignment but said it looked like the camber could be off. Returning home I discovered the tires had been aired back up to 37psi and reduced the pressure back to 26.

Next stop was a performance alignment shop that did a lot of Jeeps. They said the OEM ball joints were shot so I had performance ball joints installed. They did an alignment and said it was much better. Didn't seem any better to me and when checking psi it was back to 37. Sheesh!

Next day back to alignment shop. They said the front suspension needed more caster but to give it more would take adjustable control arms. After the new shiny gold anodized control arms were installed the car finally drove straight. Oh, yes, I did have to lower the tire pressure back to 26psi again.

What the correct tire pressure for my Jeep seems pretty subjective. This thread has been an educational exercise for me. I'll play with it and won't be afraid to air down while off road. Also, now on my list is to check out the tire jack and be certain to have one that will get a tire off the ground.

One thing the alignment shop said that stuck with me. Jeep spent hundreds of thousands of dollars engineering and perfecting the suspension. Then guys come along, add a lift kit, change all the dynamics and can't figure out why they have problems. That was a mouth full.

[Crasher]

If you're happy with 26 psi, go for it. FWIW, on a previous Avalanche, I was advised to run 36# in all positions. Instead, I weighed it and looked up the correct pressures for the weight. It turned out to be 30#. When I had a trailer on or any load, I increased the pressure for the load. It was inconvenient to do it, but I wanted to know how far the tires could run with the correct pressures. At 82,000 miles, I replaced them even though they were still safe. That experience convinced me that the "Load Inflation Table" was the only way to get the best overall performance and safety from any tire. Regardless of what a tire dealer tells you, if the pressure is not based on the weight the tire is carrying, it's it's not optimum for the best performance.

[vsheetz]

Originally Posted by vseasport

I have Nitto Trail Grapplers 35X12.50 R17.

With that tire you can likely readily run 10-15 psi when off-roading.

For dirt trail riding as you've described dropping about 25% would likely provide improved ride and be ok for street driving back to a air up point.