What the heck -- tire pressure

[jensht]

Correct psi is related to tire size and vehicle weight. It’s very simple, but seems to be difficult for so many.
Look at a chart to determine correct psi for your tire size and your vehicle weight.
https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf

Absolutely agree, the pressure should align with the weight. But there also is the vehicle driving dynamics aspect in addition to just the pure weight carrying capacity. For example, mounting a roof top tent to a car won't appreciably increase the weight, but due to the height of that weight it will increase sidewall flex in curves which can be counteracted with more tire pressure. Fuel economy, obviously can be improved to some extent with more air, ride made softer with less, etc.

At a minimum, I agree, the pressure should align with the weight, but then there are other factors and some personal preferences that play into it.

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[hookedspur.tc]

The 80 psi is at the max weight the tire is rated at. Take the total weight of your Bronco then divide by 4 and you will have your weight per tire which will be roughly half of the tire max weight rating which is also half the psi. Or just do the chalk test to avoid the math.

 

[Onestepmore]

OK, so I am very confused on some new tires (larger than OEM) and wheels I got from Tire Rack for my BD. 285/75r16 KO3 with load range E and load index 126. Tire Rack mounted and balanced the new tires on the new wheels with a Hunter Road Force, which is great. The mounted/balanced tires/wheels arrived at the garage I use at 76 PSI, which my mechanic said was way too high (and I agreed) and he put them at 40 PSI.

What has me confused is the below linked chart under "E Load Range" with my 126 load index that says the tires should be at 80 PSI (which is close to the 76 PSI Tire Rack sent them out to my garage mounted and balanced). I am guessing this for the maximum load of the weight of your truck and what you may have in it - i.e. a truck far heavier than the Bronco. I will say the 40 PSI I am at now drives great locally and on highway.

My concern/goal is a PSI that is ideal for least amount of tire wear when driving on pavement. Any thoughts/suggestions on proper PSI for a 2 dr BD with 285/75r16? Thanks!

Scroll down a bit for "E Load Range" PSI....

https://static.squarespace.com/static/5395fbd3e4b003747ed3b60a/539a2d7ce4b0854264cd356d/539a2d82e4b0854264cd419b/1378330097907/Discount Tire inflation.pdf

I've been running 39psi all the way around as the tag says. I have 20K on my 23 Bronco Badlands sasquatch and rotate and oil change every 5000. My tires are wearing identical and have had no issues...just a thought.

 

[AKBronc49]

I'm running 34psi in my E rated 315/70R17's. Happy with that!

 

[jensht]

I'm running 34psi in my E rated 315/70R17's. Happy with that!

Similar here, I've settled on 35 psi on my E rated 315/70R17. Wearing great and hasn't blown up during top speed runs on the Autobahn without any load.

 

[Montana Bronco]

I’m running my E-rated tires at 32.

 

[kodiakisland]

Absolutely agree, the pressure should align with the weight. But there also is the vehicle driving dynamics aspect in addition to just the pure weight carrying capacity. For example, mounting a roof top tent to a car won't appreciably increase the weight, but due to the height of that weight it will increase sidewall flex in curves which can be counteracted with more tire pressure. Fuel economy, obviously can be improved to some extent with more air, ride made softer with less, etc.

At a minimum, I agree, the pressure should align with the weight, but then there are other factors and some personal preferences that play into it.

Sure, the chart is a starting point that applies to most situations. The DOT/NHTSA use a factor of 1.589X or so for dynamic load ratings. I just round to 1.6X for easy math. For a vehicle with extended center of gravity I'd probably want to use a 1.8X factor or more.

Many people don't know you can exceed the weight rating of your tires as well. MPH rating goes down significantly when doing such. Goodyear used to publish an overload tire chart but I haven't seen it in years.

 

[dgorsett]

Similar here, I've settled on 35 psi on my E rated 315/70R17. Wearing great and hasn't blown up during top speed runs on the Autobahn without any load.

I too run 33 to 35 in my LREs. But the chart @kodiakisland and i assume yours to recommend that I run 44 psi to get the same load carrying as my OEM 255/75x17 SLs @ 35 psi for the reason you stated. So at 35 we should be getting all kinds of heat, wear maybe failure, but we're not, what's up? I flat tow and feel tires/wheels at each stop to check for tire/brake/bearing issues.

 

[lakesinai]

I don't have anything I can link up that supports or disproves that. However, that is what a tire engineer at Continental told me. He says that while less plies will general create more flex, higher load rated tires with reinforced sidewalks will initially resist heat build up due to less flex, but the internal friction increases and they can't dissipate the heat as well due the extra mass. That's one of the reasons why LT rated tires, even though build tougher, require more air pressure to support the same weight.

According to the tire engineer, when car manufacturers chose a tire for a car, they have a whole plethora of requirements, and the testing assumes the vehicles are overloaded and driven in the most extreme conditions, like extreme heat, and also at high speeds. So that's why many people say that the Ford recommended pressure for the Bronco is too high, since it assumes the worst.

Remember Ford had those rollover lawsuits with the Explorer years ago, related to low tire pressure. They are being cautious.

 

[jensht]

I too run 33 to 35 in my LREs. But the chart @kodiakisland and i assume yours to recommend that I run 44 psi to get the same load carrying as my OEM 255/75x17 SLs @ 35 psi for the reason you stated. So at 35 we should be getting all kinds of heat, wear maybe failure, but we're not, what's up? I flat tow and feel tires/wheels at each stop to check for tire/brake/bearing issues.

What was the OEM tire pressure on your stock tires, and where they P rated or LT tires? If P rated, then there is a conversion factor when switching to LT rated tires. What tire size are you running now, and are they LT rated tires?

Also, several years ago the door stickers for tire pressures changed in the US. They used to have both an empty load and max load rating. Now they only show the max load rating, which is why one can generally run a bit lower without issues as most don't drive with maxed out vehicles. Here in Europe they usually still have both ratings, along sometimes with an ECO pressure, and high speed pressure.

My 2nd gen Ranger Raptor had both empty (front and rear at 36 psi) and full load (front 36 / rear 39 psi) ratings. But the 1st gen Ranger Raptor, which was very similar in weight and payload only had a single pressure for both empty and loaded at 35 psi. To me that validates what I wrote earlier, that there are other factors that play into it besides load carrying capacity, but these days I doubt any manufacture would recommend a pressure below the load carrying capacity plus some sort of safety factor.

 

[crenca]

Interesting thread. It has me doing this thought experiment:

We make the run between Southern New Mexico and San Antonio TX (9 or so hours) about 2 times a year. Left lane traffic (which we are in of course) averages 87 to 95 mph (80mph speed limit), ambient temp between 105 and 110 degrees F, blacktop temp has to be significantly higher then that in the west Texas sun.

Which vehicle is less likely to suffer a catastrophic tire failure, my Bronco on OEM 315/70r17 C rated LT tires at 31psi cold, or my wife's 2016 Toyota Sequoia which currently sits on 275/55r20 "extra load" p-metric tires which are V rated (149mph) that I keep at 36-38psi cold?

The Sequoia is a serene highway cruiser, quiet, but lots of sound deadening means it sits at 6700lbs with a full tank, family and gear. The Bronco would be loud and stiff, but would be "only" 5600lbs loaded with gas, family and gear.

My gut tells that even at the lower pressure the Bronco tire has the easier time of it. I could bring the Bronco up to 34 or 35psi for a more clear advantage especially given its volumetric air capacity (the 55 sidewalls of the Sequoia approaching a lowish profile).....BUT.....the Sequoia's tires have the speed rating which ostensibly gives you a tire compound able to handle the high heat of these circumstances.

In the end, I think I trust the raw lower weight loading and higher air volume (and thus heat absorbing capacity) of the Bronco's tires over the Sequoia's manufactures claims of loading and speed rating (and thus heat capacity)....or do I? Not that we would ever take the Bronco on this trip

What are Y'all's opinions?

 

[dgorsett]

What was the OEM tire pressure on your stock tires, and where they P rated or LT tires? If P rated, then there is a conversion factor when switching to LT rated tires. What tire size are you running now, and are they LT rated tires?

Also, several years ago the door stickers for tire pressures changed in the US. They used to have both an empty load and max load rating. Now they only show the max load rating, which is why one can generally run a bit lower without issues as most don't drive with maxed out vehicles. Here in Europe they usually still have both ratings, along sometimes with an ECO pressure, and high speed pressure.

My 2nd gen Ranger Raptor had both empty (front and rear at 36 psi) and full load (front 36 / rear 39 psi) ratings. But the 1st gen Ranger Raptor, which was very similar in weight and payload only had a single pressure for both empty and loaded at 35 psi. To me that validates what I wrote earlier, that there are other factors that play into it besides load carrying capacity, but these days I doubt any manufacture would recommend a pressure below the load carrying capacity plus some sort of safety factor.

OEM size was P 255/75x17. SL @35. New is LT 265/70X17 E, I'm running 33 to 35. Calculators show to get same effective load carrying I need 44 psi.

 

[jensht]

OEM size was P 255/75x17. SL @35. New is LT 265/70X17 E, I'm running 33 to 35. Calculators show to get same effective load carrying I need 44 psi.

Yeah, that makes sense then. Since the tires aren't massively different in terms of air volume, the LT would need more pressure to match the P rated load carrying capabilities. Now you may not need the actual load carrying capacity, and prefer the tradeoff in handling vs ride comfort (I expect a significantly better ride at 33 psi than 44). So if the pressure matches the actual weight, then unless you do sustained high speed runs, you'll probably be fine.

 

[dgorsett]

Yes. I think you have explained it in a way I can understand.

It would be interesting to have a calculator to input my tire and pressure and get safe load/speed data.

 

[jensht]

Yes. I think you have explained it in a way I can understand.

It would be interesting to have a calculator to input my tire and pressure and get safe load/speed data.

Surprisingly it seems that both the European (ETRTO) and American (TRA) organizations that do these standardizations have the documents available only for paying members...Toyo still has an older chart on their website that should apply to other brands as well: https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf

You can also play around with the calculator here and change the OEM tire pressure to align with the actual weight: https://tiresize.com/pressure-calculator/

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