What the heck -- tire pressure

[BroncoBeachBuggy]

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

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[userdude]

An E rated tire at 76psi?!? I think I chipped a tooth just reading that...

Do a chalk test. I'm no tire expert but my understanding is you'd continue to use Ford's recommendation of 38psi (or is it 40psi?) if you're not comfortable doing your own thing. That's what my last tire guys did, at least, with new tires that had different manufacturer recommendations. They looked at the door plaque and insisted on using what was on there.

You can also ask why TR used that psi; my guess is to protect against flat spots in transit.

 

[ksdon]

And for your consideration, after doing a careful chalk test on our Black Diamond with Sasquatch size tires, our Mickey Thompson Baja Boss AT E rated tires run with 30 PSI front and 29 PSI in the rear. 2 Years and zero abnormal wear. This is for on pavement daily driving.

 

[CitrusBronco]

For best psi on your Bronco for tire wear do the chalk test.
As for the 75 psi you bought a E rated tire and E ratings are for heavy loads. It’s for heavy duty trucks carrying a load. They were setup and shipped for the tires rating intention not your specific use.
Broncos fit into the C load rating range.
Some people want or need features that D & E rated tires offer and run them for those features like thicker sidewalls, just not at the full load rating capacity psi.

 

[BroncoBeachBuggy]

For best psi on your Bronco for tire wear do the chalk test.
As for the 75 psi you bought a E rated tire and E ratings are for heavy loads. It’s for heavy duty trucks carrying a load. They were setup and shipped for the tires rating intention not your specific use.
Broncos fit into the C load rating range.
Some people want or need features that D & E rated tires offer and run them for those features like thicker sidewalls, just not at the full load rating capacity psi.

Yep. That's what I thought the 80 PSI was for in my OP. I wanted a KO3 and in the size I wanted it only came E rated from Tire Rack. So far driving great and just was well as my smaller and C rated OE BD tires, which I found a pleasant surprise. Will be doing a chalk test now.

 

[dgorsett]

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 always run LRE tires on my off roadable vehicles. The 80 psi is for max load as you suspect, your Bronco is not max load. Best solution is chalk test, but really you can run somewhere near door sticker. You can probably get by with a bit less since your tire is bigger and stronger than OEM. I run 33 psi on my 265/70 x 17 LRE with no abnormal wear. I'm sure you'll be fine somewhere between 30 and 35. There are calculators online, but I find them a bit wonky to use.

 

[tock13]

Interesting thread for sure. I’m not a tire expert by any means but I’d be pi$$ed at having to pay extra shipping on all that unnecessary air.

 

[crenca]

An E rated tire at 76psi?!? I think I chipped a tooth just reading that...

That's even well past 'Obama' pressure

I run my Wranglers (315/70x17 - Sasquatch OEM's) at 31 psi cold for a little more cushy ride then 37 or whatever the door jam recomends. I suppose if I was going to be doing 90mph on the highway for a few hours I might bring them up to 33 or 34.

As long as your tires/vehicle are in good shape, modern recommendations of 35psi + for light passenger vehicles are all about fleet fuel mileage targets and not about the consumer who actually have to use/pay for the consequences...

 

[jrollo7]

Interesting thread for sure. I’m not a tire expert by any means but I’d be pi$$ed at having to pay extra shipping on all that unnecessary air.

 

[SierraBronco]

An E rated tire at 76psi?!? I think I chipped a tooth just reading that...

Do a chalk test. I'm no tire expert but my understanding is you'd continue to use Ford's recommendation of 38psi (or is it 40psi?) if you're not comfortable doing your own thing. That's what my last tire guys did, at least, with new tires that had different manufacturer recommendations. They looked at the door plaque and insisted on using what was on there.

You can also ask why TR used that psi; my guess is to protect against flat spots in transit.

Yip! With our heaaavvvvyyyy 40’s but still load range C we got down to 26 psi before getting what felt like the correct pressure. Check the chart and still good to go.

Was picking up a dually one time and told them I wanted the fronts at 55 and rears at 45. Show up and they’re all at 80. “if we lower it the TPMS will go off.” Well, it won’t, but also I’m not hauling anything home. I don’t need max pressure. Gonna be like driving a brick!

 

[jensht]

This website has a convenient calculator that will show you what pressure has the equivalent load rating as your original tires.

https://tiresize.com/pressure-calculator/

The problem with the chalk test is that it doesn't account for heat build up. Especially an E rated tired on a lighter vehicle like the Bronco will let you run a low pressure by the chalk method, but in reality if you are driving at higher speeds, or higher sidewall loads, the heat build up might be higher than it should be. Now I know that a ton of people will come and say that they never has an issue with the chalk method, and who knows, maybe that is the "best" method. But if you want to be around the same specs that the manufacturer of the tires and truck intended on the original size, then this will get you in the same area. I would start there, and then fine tune based on your needs and desire.

You'll also notice that for an LT rated tire the website won't go below 35 psi. That is due to the fact the the tire manufacturers won't recommend a lower pressure on LT tires due to their thicker sidewall, compared to a P Metric tire, as that causes more heat build up, especially with a heavy load or towing.

I am not surprised that Tire Rack inflated them to the max rated PSI. Keeps their liability down when someone mounts them on a heavy duty truck and loads 3000lbs in the back.

 

[crenca]

That is due to the fact the the tire manufacturers won't recommend a lower pressure on LT tires due to their thicker sidewall, compared to a P Metric tire, as that causes more heat build up,

Interesting, I hadn't thought about this that more plies = more, not less, heat build up due to repeated flexing (of the rolling tire). Do you know if this is a fact verified in some way?

 

[jensht]

Interesting, I hadn't thought about this that more plies = more, not less, heat build up due to repeated flexing (of the rolling tire). Do you know if this is a fact verified in some way?

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.

 

[SierraBronco]

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.

It is pretty wild to see what goes in to the construction of a tire. Also interesting to think about how the tire flattens as it rolls down the road. Seems obvious enough, but to put it into a perspective that as you roll the tire is constantly being put into a state of deflection it really makes you appreciate what the sidewalls go through-particularly on the high speed stuff. There’s a video on YouTube where a guy put a GoPro inside the tire to watch how the balance beads work, and it’s cool to see the sidewall deflection from that perspective.

 

[kodiakisland]

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

The load rating of the tire does not determine the correct psi.

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