Tire pressure on 35s : 39 psi vs 35 psi

This is such an interesting question, actually it's the answers that are interesting.

Higher pressure means less contact patch. It means less heat in the tire. It means better gas mileage.
Higher pressure means harder sidewall (more pressure), so a stiffer (less comfortable) ride. Higher pressure means (sometimes) more wandering.

Lower pressure means more contact patch. It means more heat in the tire. It means worse gas mileage.
Lower pressure means softer sidewall (less pressure), so a softer (more comfortable) ride. Lower pressure means (sometimes) more wandering.

People will sometimes get very concerned about low pressure creating heat, and causing tire damage (look up Firestone tire separation, or Ford Explorer Firestone claims). People will sometimes get very concerned about ride quality and high/low pressure. People will sometimes say the placard on the door jamb rules. People will sometimes say that's the max pressure and you can go much lower. People will sometimes say you need to put a chalk line across the tread and drive around and see where it wears off, then adjust pressure to create an even contact patch. People will say the static and moving contact patches are different. People will say the pressure will change after driving, or while sitting in the sun, or on one side versus the other, so you should inflate to a cold temp and realize it will rise by a couple pounds on long drives, but not enough on short ones.

Broncasaurus

This may be an "I can't believe he just asked that moment." I'm asking it anyway.

I've seen a few threads in various forums where people have discussed lowering the pressure in their tired from Ford's recommended 39 psi to ~25 psi. In fact, when I picked up my Bronco, the salesman mentioned that one of the employees at the dealer did just that on their Sasquatch. I understand that people are reporting that their tires ware more evenly at the lower pressure. I also saw one person say that they felt their ride was "smoother."

My question is why did Ford pick 39 psi? How is that determined by a manufacturer? Are there any drawbacks to running the lower pressure? Are there times when someone that is normally running at a lower pressure should go back to the recommended values?

Thanks for sharing your thoughts on this. I'm ready to learn something.

Vehicle manufacturers select tire pressures based on a number of parameters. First is fuel economy, second priority is handling, third is GWV and tow rating, next is usually ride, fifth is tire noise. Of course, lower tire pressure improves ride, but it also decreases coast-down in increases tire rolling resistance. BTW, I am a retired Ford 34 year truck engineer.

Mikey Smith

Vehicle manufacturers select tire pressures based on a number of parameters. First is fuel economy, second priority is handling, third is GWV and tow rating, next is usually ride, fifth is tire noise. Of course, lower tire pressure improves ride, but it also decreases coast-down in increases tire rolling resistance. BTW, I am a retired Ford 34 year truck engineer.

It's interesting that you were an engineer at Ford. With your background, do you opt to follow the recommended inflation value or do you do something different? Do you feel like the recommended value is a good compromise of all those concerns? How do things like All Season vs MT tires impact those calculations?

When I left the dealership, it felt like I could feel every little ripple in the road. Started playing with all the instrument panel pages and came across the tire pressure page. It said I had a little over 50lbs on all 4 tires. I checked the door sticker which said 39lbs so I reduced them all. The ride definitely got much better. Now I'm not sure if I'm keep them at 39 which should get me better gas mileage or lower them a little so it smooths out the ride a little.

while I don't have my Bronco (yet), I have been driving a similarly-capable truck for the past year and a half (Chevy ZR2 Bison), and have done some playing around with tire pressures - there is absolutely a difference in ride quality for daily driving in terms of the stiffness of the ride, based on tire pressure. I think my truck also recommends 39psi, but I've found that even dropping to 34 psi makes a difference in the ride quality, with negligible impact to MPG. When I'm off-roading, I usually drop to about 18psi to help with ride quality and help prevent chances of puncture, as well as increased traction.

Broncasaurus

It's interesting that you were an engineer at Ford. With your background, do you opt to follow the recommended inflation value or do you do something different? Do you feel like the recommended value is a good compromise of all those concerns? How do things like All Season vs MT tires impact those calculations?

Decent question. The implication of migrating from example to All Season from Mud Terrain is not important is maintaining the maximum load index and speed ratings, plus temperature grading. If you really want to get into tire construction, just Google "DOT Tire Identification Number". That is what most reputable tire stores use to ensure the replacement tire they put on your vehicle is legally and technically equivalent to production.

I run my SAS tires at 34 cold and after a few miles of highway driving the reach 38-39 and seems to be a good compromise for me.

After significant experimentation over the past 9 months and 50/50 mix of highway/local roads in regular drive mode in a Badlands Sasquatch w/2.7L & 35” Goodyear rubber:

At 41 psi MPG is +1 better than the recommended 39 psi and +2 compared to a much nicer on-road 36 psi setting. YMMV

Bschurr

After significant experimentation over the past 9 months and 50/50 mix of highway/local roads in regular drive mode in a Badlands Sasquatch w/2.7L & 35” Goodyear rubber:

At 41 psi MPG is +1 better than the recommended 39 psi and +2 compared to a much nicer on-road 36 psi setting. YMMV

Dang, that is more than I would have expected. Thanks for the info.

I had about 43-45 psi in mine when I got. Took her off-road when she was just 2 weeks old and I aired down to about 15-20 psi. Then when I aired back up, only went to 35. That was Jan 22nd. Noticed several mpg loss on gas mileage. Then a few weeks ago, my husband said that my tires were wearing on the outside and have an alignment done when I took her in for an oil change & rotation - I took it to the Ford Quick Lane tied to the dealership. They said the alignment was spot on, tires were out of balance. I asked what may have caused the tire wear and he said too low psi can do that. Not sure if I believe that. They are now at 40-42 - which is where I aired them up to when the gas prices got really crazy. I prefer a higher psi to help keep tire wear to a minimum and forgo my comfort or ride quality. And, these are some noisy tires too, imo

Deano Bronc

When I left the dealership, it felt like I could feel every little ripple in the road. Started playing with all the instrument panel pages and came across the tire pressure page. It said I had a little over 50lbs on all 4 tires. I checked the door sticker which said 39lbs so I reduced them all. The ride definitely got much better. Now I'm not sure if I'm keep them at 39 which should get me better gas mileage or lower them a little so it smooths out the ride a little.

Same. Each tire was 50 or 49 when leaving the dealership. Noticed on the ford pass app while playing around. Next morning checked them all and dropped them down to 39. Extra hot in Texas right now, so I might drop them down to 35 psi until I replace with 37s.

Bschurr

After significant experimentation over the past 9 months and 50/50 mix of highway/local roads in regular drive mode in a Badlands Sasquatch w/2.7L & 35” Goodyear rubber:

At 41 psi MPG is +1 better than the recommended 39 psi and +2 compared to a much nicer on-road 36 psi setting. YMMV

So you have improved the fuel economy rolling resistance metric, and increased what us engineers call coast-down time. Handling likely improved a bit, but tire wear in the center may take a minor hit.

Tell me you are an engineer without telling me you are an engineer…
:cool:

Mikey Smith

So you have improved the fuel economy rolling resistance metric, and increased what us engineers call coast-down time. Handling likely improved a bit, but tire wear in the center may take a minor hit.

Sven

I run my SAS tires at 34 cold and after a few miles of highway driving the reach 38-39 and seems to be a good compromise for me.

I'm going through this now during this Chicago cold. I brought my tires down from 39 to 34 and they all rise, pretty consistently, past 40 psi after a short time of driving.

Proper pressue is simple to find, measure tire temp across the tread and adjust until you chave equal reading on the left, center and right side. Start there. Under inflation and over inflation on the road are both dangerous.

I adjust my Nitto Rodge Grapplers based on usage. 20 for sand, 35 for on off use, 45 for daily driving and 50 for long trips. The max pressure is around 70-80 for this tire however and I have never run them that high as 50 is plenty hard and a bit loose on a wet road. Watch for tread wear signs and rotate often. Good luck

39 is too hard and makes the truck ride hard. 34-35 is fine. At speed they warm up to 39, better ride. At 27k miles tread wear is even and near perfect. I got 10k more on these tires for sure.

Everybody running lower tire pressure, doesn't your low tire pressure warning light come on?

I find the recommended tire pressure is where you start.
As stated earlier, many factors go into the recommendations. My 350 has a pressure for it's max gvw. If it's driving around empty, lower pressure is okay.
Taking tire temperature can help with setting pressure too.
I was working at Firestone during the explorer/tire problem.
The tire needed higher pressure for vehicle stability. That also required a specific tire. It looked like and had the same name as a popular tire but shame on you for putting it on.