MT Baja Boss AT 255/85 R17 - MT recommends 52-55 psi

[userdude]

I hear about these manufacturer inflation tables, specifically Toyo's:

https://www.toyotires.com/media/pxcjubjs/application_of_load_inflation_tables_20200723.pdf

Sponsored

 

[GA2Dr]

Here is an excellent load inflation calculator from Performance Tire website that agrees with the spreadsheet calculations from TRA:

Inflation Pressure vs Load Calculator

Notes

• This tool assumes a linear load-to-pressure relationship up to the reference pressure (TRA table anchor). Real tables are stepped; use this as an estimate.
• Do not exceed the tire’s sidewall max pressure or the wheel’s PSI rating.
• Under-inflation causes heat build-up and potential failure. A change of ~5 psi can significantly alter load capacity.

Hope others find this info useful and helpful.

This is helpful, thanks.

According to this calculator I should be 42psi cold which I can live with. I started at 40 and everything seemed fine and honestly the difference between 40 and 33 is not huge in terms of ride. I'm going to call M/T today too to see what they say.

 

[sandersd]

I just spoke with Ben at MT tech dept and he said the door placard weight is not the weight used to calculate the loading on the tires, but it is the total vehicle weight + passengers + cargo + towing. He said a lot of people have the misconception (including me) that the GVWR is the number to use and this causes confusion. He explained that door listed number for my Bronco is 5860 lbs, but the actual number used for the load calculation is a little over 10,000. The 55 psi for the 255s makes more sense when that is taken into consideration. He also stated the 255s should have never been mounted on 8 1/2" rims since they are not designed to be used on a rim larger the 8".

He said they have a Badlands SAS there with the 315s load range E that they drive across country and the tires are comfortable at the standard 39 psi, which is what they recommend. He also stated they cannot recommend anything other than what the door says for legal reasons, irrespective of anyone who called and received different info.

I'm glad I looked into this and feel I will be very happy with the 315s and have a greater understanding of tire selection criteria to boot.

 

[sandersd]

This is helpful, thanks.

According to this calculator I should be 42psi cold which I can live with. I started at 40 and everything seemed fine and honestly the difference between 40 and 33 is not huge in terms of ride. I'm going to call M/T today too to see what they say.

One thing different for you is your rim width is 7.5" (mine is 8.5") which is within spec for the tire. The primary concern is not wear but heat loading. Tire rubber degrades at 200 degrees and at 250 the structure starts to fail. Hopefully overheating will not be the case for your setup. They wouldn't go into any more detail regarding this since they said the tire should never have been mounted on my rims.

Be sure and let us know what they say regarding the pressure you are running at.

 

[GA2Dr]

I just spoke with Ben at MT tech dept and he said the door placard weight is not the weight used to calculate the loading on the tires, but it is the total vehicle weight + passengers + cargo + towing. He said a lot of people have the misconception (including me) that the GVWR is the number to use and this causes confusion. He explained that door listed number for my Bronco is 5860 lbs, but the actual number used for the load calculation is a little over 10,000. The 55 psi for the 255s makes more sense when that is taken into consideration. He also stated the 255s should have never been mounted on 8 1/2" rims since they are not designed to be used on a rim larger the 8".

He said they have a Badlands SAS there with the 315s load range E that they drive across country and the tires are comfortable at the standard 39 psi, which is what they recommend. He also stated they cannot recommend anything other than what the door says for legal reasons, irrespective of anyone who called and received different info.

I'm glad I looked into this and feel I will be very happy with the 315s and have a greater understanding of tire selection criteria to boot.

Well now I'm confused. My understanding is that the GVWR is the maximum weight limit for safe operation of the vehicle. That's with people, cargo, fuel, everything. I believe the GVWR does not take into account towing, but putting towing aside why are they doubling the GVWR? Or is all that extra weight only to account for towing? If that's the case I'm not concerned because I'm never going to tow anything with my Bronco (and if I do I can air them up to 55psi). Am I missing something?

Agreed on the rim issue and I don't have that issue as mine are mounted on my factory BD alloys that are 7.5" wide.

 

[Brian_B]

He explained that door listed number for my Bronco is 5860 lbs, but the actual number used for the load calculation is a little over 10,000.

Yeah I don't get this either. The number on the door is the max safe weight from the manufacturer, not the actual weight. And then just seemingly arbitrarily making that a much larger number anyway. No wonder they are coming up with 55psi - they think you are driving a dump truck or something.

Also, you mentioned that tire manufacturers will defer to the door placard. That's limited to a particular tire size, but it makes no regard for tire manufacture - just a particular size and load range. So... if Ford can somehow come up with one single pressure that works across pretty much every make and model of tire out there (limiting it to a specific size and load range) - I think that says something. I'm not sure exactly what yet... but it's informative and pertinent to this somehow.

But I do agree with many above - the load tables you refer to are excellent. Chalk is just a layman's tool, something quick and easy you can do in a driveway that will get you a good contact patch. You are correct, there's no guarantee that the tire manufacturer intended for that to be the actual contact patch. But, as a general rule of thumb, it has held up pretty well for a long time and proven to be a pretty good method to maximize tire longevity. But if you try to take a tire in on warranty because it wore early or blew up, and you try to use "But I chalk tested it" as your defense - you will not win that argument. But if you match the door placard (in all respects, pressure/size/rating) or can make a semi-informed case with these load tables as reference, then you have a really strong leg to stand on.

Looking at the Toyo chart - LR C's may start to get in trouble at pressures below 30, LR D and up though are pretty safe all the way down to 26 listed on the chart -- on a Bronco, using GVWR (I realize I'm drawing a broad generalization, so with the usual caveats - this is just one particular tire and not indicitive of all tires, different sizes have different numbers, etc etc). I am assuming this is weight on a tire — so axle weight divided by two - and not total vehicle weight - listed on the tire chart.

 

[userdude]

No wonder they are coming up with 55psi - they think you are driving a dump truck or something.

Say 4dr without saying 4dr, just sayin'!

 

[Brian_B]

Even if you consider towing in there - max tongue weight is only 300lbs or so. Not the entire 3,500 trailer weight

 

[ducksface]

Tires aren't chalk as an end-all argument.
They're chalked to remove the butt covering legalities thrust upon us by a Jr lawyer in a cube farm at the tire company and then compounded by the jr lawyer at ford.

The tire engineer who supposedly gives you that chart knows no more than the oil filter engineer you wish would choke on a bone.

The computer says, the lawyer interprets, another adds to it...and you have a chart.

That's the reason for chalk.
You
Your usage
Your scenario
Chalk tailors to you.
It doesn't insist on compliance all the way down to a single psi.

And(though i dislike ai, it gives a reason to ponder):

An individual tire on a non-commercial vehicle in the United States has an estimated annual blowout probability of roughly 0.007% to 0.018% (or roughly 1 in 5,500 to 1 in 14,000), which translates to a per-car annual risk of 0.028% to 0.071%.
When factoring in total mileage, a tire experiences a blowout once every 190 million to 500 million miles driven per individual tire. [1, 2]

Or
Eye-caliper as has been done for 100 years.

In this thread we've learned tires should be anywhere from 26 to 52 psi.

 

[23OBX2.7]

Yeah I don't get this either. The number on the door is the max safe weight from the manufacturer, not the actual weight. And then just seemingly arbitrarily making that a much larger number anyway. No wonder they are coming up with 55psi - they think you are driving a dump truck or something.

Also, you mentioned that tire manufacturers will defer to the door placard. That's limited to a particular tire size, but it makes no regard for tire manufacture - just a particular size and load range. So... if Ford can somehow come up with one single pressure that works across pretty much every make and model of tire out there (limiting it to a specific size and load range) - I think that says something. I'm not sure exactly what yet... but it's informative and pertinent to this somehow.

But I do agree with many above - the load tables you refer to are excellent. Chalk is just a layman's tool, something quick and easy you can do in a driveway that will get you a good contact patch. You are correct, there's no guarantee that the tire manufacturer intended for that to be the actual contact patch. But, as a general rule of thumb, it has held up pretty well for a long time and proven to be a pretty good method to maximize tire longevity. But if you try to take a tire in on warranty because it wore early or blew up, and you try to use "But I chalk tested it" as your defense - you will not win that argument. But if you match the door placard (in all respects, pressure/size/rating) or can make a semi-informed case with these load tables as reference, then you have a really strong leg to stand on.

Looking at the Toyo chart - LR C's may start to get in trouble at pressures below 30, LR D and up though are pretty safe all the way down to 26 listed on the chart -- on a Bronco, using GVWR (I realize I'm drawing a broad generalization, so with the usual caveats - this is just one particular tire and not indicitive of all tires, different sizes have different numbers, etc etc). I am assuming this is weight on a tire — so axle weight divided by two - and not total vehicle weight - listed on the tire chart.

Most of the low pressure risk for daily is sudden directional changes where side wall folds, or breaking the bead or pinch flat in extreme cases.

I really want to see the Moose test with Broncos...wonder how many times it will roll...

I perosnally like E Destination XT thatvare great in winter and quiet, but since want another set of tires on other wheels a C or D will be worth a try.

 

[ducksface]

Most of the low pressure risk for daily is sudden directional changes where side wall folds, or breaking the bead or pinch flat in extreme cases.

Exactly.
Safety rims have been federal law since about 1953.
That's where the 'no beadlocks' thing comes from...a 1953ish law.

I've never seen a passenger size gator on the road. I doubt 1:100,000 people have seen them in any number worthy of anything other than to think of the rarity.

 

[sandersd]

I certainly agree it is confusing, but that is what I was told. They don't publish the load and inflation data or their methods for determining their numbers.

That's why I decided to swap for the OEM size and avoid the issue entirely. I have more important things to spend time worrying about.

 

[ducksface]

I certainly agree it is confusing...

I have more important things to spend time worrying about.

Exactly.
Its's a worry for the sake of worry. The most minimal of value.

 

[GA2Dr]

One thing different for you is your rim width is 7.5" (mine is 8.5") which is within spec for the tire. The primary concern is not wear but heat loading. Tire rubber degrades at 200 degrees and at 250 the structure starts to fail. Hopefully overheating will not be the case for your setup. They wouldn't go into any more detail regarding this since they said the tire should never have been mounted on my rims.

Be sure and let us know what they say regarding the pressure you are running at.

So I think you got some bad info from the tech you spoke to at M/T, which tracks with what he told you about the weight he was using to calculate. I just called M/T and talked to a guy named Mark. He did not ask for GVWR, he asked me for the original size tire, my new tire size, did some calculations and told me the recommended cold tire pressure would be 36psi.

I will probably bump back up to 38psi, which is somewhere between what M/T recommended and what the online calculators recommended.

I think it's fair to worry about running your tire under inflated, I think others who have chimed in saying that the occurrence of a blowout is about the same as getting hit by lightning also make a good point. OP also had the added issue of the rim being wider than what M/T says is proper which I do think is a legit concern.

Hopefully all this information is helpful for those running the 255/85/17 Baja Boss.

 

[sandersd]

Three different techs, three different answers. Whom does one believe? That’s why I gave up and went OEM size. I hope it works put well for you.

Sponsored