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

[87-Z28]

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.

@kodiakisland. Curious what your thoughts are on this?

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[sandersd]

Will a bigger tire will have the same problem?

The larger size is the same as the OEM, but I have a similar question and intend to call tomorrow and ask about the pressure and loading for the OEM size in a load range E or D as opposed to a C. I don’t want a tire where there is a conflict in the design pressure vs the pressure for safe use, even tread width, and longevity.

 

[sandersd]

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.

Tire pressure can certainly be adjusted to even tire wear, but my overriding concern is running a tire under inflated, that is, significantly below the pressure it is designed for, could result in overheating the tires and causing early failure. A failure at speed, and everything that goes with it, is something I want to do my best to avoid.

 

[GA2Dr]

Tire pressure can certainly be adjusted to even tire wear, but my overriding concern is running a tire under inflated, that is, significantly below the pressure it is designed for, could result in overheating the tires and causing early failure. A failure at speed, and everything that goes with it, is something I want to do my best to avoid.

Yeah that's an interesting point. M/T's website says if you're running a non-OEM tire you should call them, which is what you did. I think I'll give them a call and see if they advise the same pressure. Looks like you're 2 dr - what info did they ask when you called? GVWR?

 

[akturbo]

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

OP said they were going back to stock size, so they should consider the 35x12.5 LR D instead of the 315 LR E.

 

[BilletWilson]

Im running these in 37". Gave MT my front and rear axle weights and they recommended 25 psi at all 4 corners. I ran 7 sets of NRG's so this is a pretty big change from what I've been use to. While the NRG tread is basically flat, the MT's are curved towards the edges. MT said there is a break in where it will take 500-1,000 miles for the rubber to relax on the outer edges. They said NOT to adjust psi to make a perfect contact initially until they fully break in.

 

[Brian_B]

The larger size is the same as the OEM, but I have a similar question and intend to call tomorrow and ask about the pressure and loading for the OEM size in a load range E or D as opposed to a C. I don’t want a tire where there is a conflict in the design pressure vs the pressure for safe use, even tread width, and longevity.

Will be interesting to hear what they say.

 

[TruckCamper]

Chalk is a BAD way to set tire pressure!

Tires are designed to carry certain loads at certain pressure. This is especially true at higher speed 50mph or more.

In general higher ply count tires will need more pressure for the same load.

Tire blowouts are most often due to tire pressure being too low. Read up on how TPMS Sensors came to be mandatory.

Also consider the Firestone blow outs in the 90s. Studies showed it was due to too low of pressure, too high speeds and mostly failed in high temperature geographic locations

 

[Squatch]

Also consider the Firestone blow outs in the 90s. Studies showed it was due to too low of pressure, too high speeds and mostly failed in high temperature geographic locations

That's awfully conclusory--Firestone would certainly hire you.

Firestone and Ford went about blaming each other. Ford recommended 26psi and Firestone said it should've been 30psi, but Firestone warranteed the tires at 26psi for 11yrs. (Wikipedia for whatever its credibility may be)

The major issue was tread separation which did occur in high temperature environments like Arizona and Saudi Arabia. Low pressure in a tire can increase tire temp, it was a factor, but research found that among factors "vehicle load always dominated" and the NHTSA determined it was a design defect. The lower recommended psi is thrown out there as a likely reason why tires on Explorers were more prone to tread separation than Rangers, but they didn't really go into it.

One disturbing correlation is the tense union negotiations at the Decatur plant and higher failure rates of tires produced in Decatur according to research by Princeton and the National Bureau of Economic Research.

 

[sandersd]

Im running these in 37". Gave MT my front and rear axle weights and they recommended 25 psi at all 4 corners. I ran 7 sets of NRG's so this is a pretty big change from what I've been use to. While the NRG tread is basically flat, the MT's are curved towards the edges. MT said there is a break in where it will take 500-1,000 miles for the rubber to relax on the outer edges. They said NOT to adjust psi to make a perfect contact initially until they fully break in.

That is interesting. None of this was mentioned in my conversation with MT. I hope I get a different tech when I call today.

 

[sandersd]

Yeah that's an interesting point. M/T's website says if you're running a non-OEM tire you should call them, which is what you did. I think I'll give them a call and see if they advise the same pressure. Looks like you're 2 dr - what info did they ask when you called? GVWR?

They need per axle weight from the placard on the door panel.

 

[sandersd]

Chalk is a BAD way to set tire pressure!

Tires are designed to carry certain loads at certain pressure. This is especially true at higher speed 50mph or more.

In general higher ply count tires will need more pressure for the same load.

Tire blowouts are most often due to tire pressure being too low. Read up on how TPMS Sensors came to be mandatory.

Also consider the Firestone blow outs in the 90s. Studies showed it was due to too low of pressure, too high speeds and mostly failed in high temperature geographic locations

Agreed. This is from the FAQ on the MT site regarding tire pressure:

PROPER INFLATION PRESSURE
Again, the pressure listed on the tire sidewall is the maximum pressure and not intended as a guide for normal inflation pressure. These things are dictated by the vehicles weight and use (ie: high speed). Always start with the vehicle manufacturer’s information on inflation pressures. With modified vehicles, or vehicles using tire sizes other than OE, it is valuable to understand the weight bias of the vehicle and intended use along with the maximum load capacity and inflation.

Always use a tire with a load carrying capacity that matches or exceeds the weight carrying capacity of the OE tire. With light truck tires and applications, the load capacity is more critical than the load range when choosing non OE sizes. Due to the reduced load capacity on P-metric tires, Mickey Thompson does not recommend their use on O.E. LT tire applications.

It is also important not to check tire pressure when the tires are hot after use. Over-inflation can produce a harsh ride, makes the tires susceptible to impact damage, and promotes faster than normal tread wear along with decreased traction performance. Under-inflation (or over-loading) will cause excessive heat build up which can lead to tire failure.

For proper tire inflation on Mickey Thompson Tires, please contact M/T Tech Department at 800-222-9092 (Option 5) with original tire size and stock tire pressure. (The front and rear stock tire pressure can be found on the driver door placard/sticker)

 

[sandersd]

This statement from the FAQ is what originally bothered me and prompted this thread:

"Always use a tire with a load carrying capacity that matches or exceeds the weight carrying capacity of the OE tire. With light truck tires and applications, the load capacity is more critical than the load range when choosing non OE sizes."​

With the 255 tires and a lower pressure of around 30 psi, the load carrying capacity of the tire is reduced. MT doesn't publish their load pressure tables, but from the one at Toyo is can be seen that such a reduction substantially reduces the tires capacity, potentially below the per wheel load on each tire on the Bronco. Without access to the tables, I have to rely on what they tell me. What they told me led me to go back to the OEM tire size, though I have questions I will posit today regarding the load range (E vs C) and it's suitability in this application.

One encouraging note is both Discount tire and Tire Rack list the LT315/70R17 Load Range E tire as a direct replacement for the stock tire, which is a load range C.

I also note that I could have saved $500 (including taxes, road hazard, shipping and installation) by buying the same tire from Tire Rack.

 

[sandersd]

I found this really useful calculation spreadsheet on the Tire and Rim Association website.

LT High Flotation Profile

It allows one to enter their tire size and calculate the load capacity at different pressures.

You can find it by going to the TRA webpage and searching for "Load inflation tables and approved rim contour charts".

This file is for high flotation tires:

There is also a version for metric sized tires and has the LT315/70R17 selection:

FYI it is MS Word and doesn't display properly in Mac.

 

[sandersd]

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.

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