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

[sandersd]

I just replaced my Goodyear LT315/70 R17 with Mickey Thompson Baja Boss AT 255/85 R17 E. I went to the source for tire inflation recommendations for normal highway use. I gave the tech rep I spoke with the stock tire and wheel specs along with the weight and loading for the Bronco and she calculated a recommended pressure of 52-55 psi for the new MTs. I told her I found that difficult to believe so she checked by calculating it several different ways as well as consulted with others in the tech department and came up with the same recommendation based on the weight and loading from the Bronco. Apparently, as the tire is narrower it needs more pressure to carry the same load as a wider tire.

I still find it difficult to believe the recommended pressure will not result in the center tread wearing much more quickly than the outer 1/3 of tread. I have the pressure at 40 psi for the moment and it looks like this:

The rolling resistance is noticeably less (I was surprised by how much) than with the wider tires and turn in is much easier. I didn't notice any squirreliness at freeway speeds.

Maybe this is a good pressure or maybe it isn't for tread wear, I guess I'll see as the miles go by. A concern is that with the lower pressure the tire is rated for less loading, less than the weight of the Bronco. I'm not sure how that affects the safety and handling as well as tire life expectancy. An additional variable is the recommended wheel size is 6.5"-8", and the SAS rims are 8.5". It's only 1/2" so I expect the effect will be nominal, but still it is outside the tire's design parameters. (I guess I read the wrong line in the chart for wheel width or I probably would have just went with the same 315 width.) Also, it seems counterintuitive that a tire with a heavier load range requires a higher pressure, but I don't know what the variables are that MT uses in their calculations.

I queried ChatGPT and at first it agreed with me, but when I input the tech departments info, it agreed with MT:

"Run 52 psi cold initially, rather than immediately using 55. Then collect objective evidence:

​

Measure tread depth across each tire now with a gauge—at the inner shoulder, inner intermediate rib, center, outer intermediate rib, and outer shoulder.

1. Repeat every 1,000–2,000 miles at the same locations.
2. Rotate frequently, probably every 5,000 miles.
3. Watch steering behavior, wet braking, ride harshness and pressure rise.
4. Do not reduce pressure substantially based only on a chalk test."

I don't really trust ChatGPT unless I can independently verify its findings. I work in engineering so I have researched this from an engineering perspective, but it is difficult to find variances from tire manufacturer's recommendations, I assume due to liability concerns. I've only found anecdotal references.

If anyone has been running the 255/85 R17s on the SAS wheels (or other narrower wheels) can you please share what psi you are maintaining, what the wear pattern looks like and how many miles you have on the setup ? I would greatly appreciate your input.

Thanks for your time and input.

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[Brian_B]

Just looking at your pic you can tell it's gonna wear center heavy - look at the dust imprint on the tread. The outside of the outer lugs are not even touching the pavement.

LR E's may want a bit higher pressure, but I would still chalk test them and see if you are after tire longevity. I bet 50+ PSI is ~awesome~ for MPG though.

Chalk test is essentially what your pic is showing with dust, just a bit more controlled. You want an even pattern completely across the contact patch. Overinflated and the edges will not show any chalk wear (or the inverse, will show no dirt picked up from pavement). Underinflated and the center will be weak.

 

[userdude]

You might consult your dental insurance first.

 

[HighVelocity]

Those tires are meant for lifted duallys. You're not going to want anywhere near 55psi.

Looking at the dirt pattern on the tire, you're overinflated. I'd bring it down to 40psi and work from there.

Chalk patterning will get you perfect.

 

[sandersd]

I'm at 40 psi now. I used several calculators online and get anywhere from 28 to 47, although I don't put too much confidence in calculators unless I know where their calculations come from (one used a formula by the Tire and Rim Association).

My plan is to try a few different pressures and take tread depth measurements at intervals to ascertain actual wear. Probably do the chalk test too as it can't hurt.

In the meantime I'm hoping drivers with the 255/85 tires will chime in.

 

[sandersd]

After further research and consideration I have decided to swap these tires for the OEM size of 315/70. While the wear issue could be resolved with adjusting tire pressure, the issue of tire overheating due to operating at a lower pressure than the tire is designed for remains and is something I am not willing to take a chance on. A catastrophic tire failure would be, well, catastrophic at speed.

Thanks for your comments.

 

[telenerd]

Why E rated tires? Those are gonna ride rough onroad and off-road regardless of air pressure.

 

[sandersd]

Because they only make it in an E.

 

[akturbo]

Because they only make it in an E.

Look at the Baja AT Load Range D non-metric sizes. I'm running 35's and 37's Ds on our Broncos.

 

[ducksface]

My bronco is running stock 35' tires at 30psi due to a chalk test. I might go lower.
My slightly heavier blazer runs 26psi on 31.5x11x15(?) falken at3.
My 3/4 ton avalanche runs at 33psi on 16"x34(ish)

Chalk the tires.

 

[sandersd]

I thought you were referring to the 255s.

 

[EasternSierra]

Look at the Baja AT Load Range D non-metric sizes. I'm running 35's and 37's Ds on our Broncos.

The only Baja Boss pizza-cutter is the 255/85R17 in load range E.

 

[zyglyrox]

shame about how you swapped back, the look would some high offset wheels would have a KILLER classic look.

 

[GA2Dr]

I am running these. Have about 6,000 miles on them. Started at 40 psi cold and have dropped them down to about 33 psi cold. No chalk test, not scientific, just based on ride and wear pattern.

 

[Brian_B]

After further research and consideration I have decided to swap these tires for the OEM size of 315/70. While the wear issue could be resolved with adjusting tire pressure, the issue of tire overheating due to operating at a lower pressure than the tire is designed for remains and is something I am not willing to take a chance on. A catastrophic tire failure would be, well, catastrophic at speed.

Thanks for your comments.

What makes you think a bigger tire will have any different of a problem?

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