Tactical Driver,
First, the link I provided is for MATERIAL fatigue. Fatigue applies to ALL structural materials, including rubber. It's just that there are many really good examples from the metal working industry - and in fact, that is where the first discoveries of fatigue were made. The principles are the same regardless of the material in question.
No, rubber fatigue is not chemical deterioration. It is a Physics phenomenon. Chemical deterioration does indeed take place in rubber - and it is both heat and age related - and it will affect the point of failure and the mode of failure. But fatigue also plays a role in both when the failure takes place, what part of the tire fails, and when it takes place. To say this is complex just scratches the surface the problem.
How do I know it is a truck tire? The smarta$$ answer is: Anyone who has been in the tire business as long as I have been will be able to tell that.
But the evidence is that the rim is a truck type rim. It has a large amount of offset and a 15° bead taper - which was never used in passenger car tires and hasn't been supplied OE for light trucks for over 20 years. The rim is a steel rim (not aluminum alloy) with a spider configuration common for truck type rims. Further, the tire has a deeper tread depth than is used in either passenger car tires and light truck tires. It has a high aspect ratio (on the order of 85%), which precludes it being a passenger car tire.
20% to 25% of the burst? Well, the sidewall of tires will give a maximum inflation pressure - and since the burst pressure is NOT a specification, you would need to know what the burst pressure is of the particular tire under study. I've done the calculation for the strength of the sidewall plies as part of a training exercise when I first started in the business - and while I forget the exact value of the factor used in the calculation (and it would be a trade secret for every company), there is indeed a factor.
You can work this out yourself, by looking at the SN curve in the link I provided. While the curve in the Wikipedia article is for aluminum, SN curves are similar for most materials. Just an FYI, 50,000 miles is on the order to 10 million cycles, so you have to compare the value at 10 milllion cycles to the value for a single cycle (the largest value). If you do that, you'll get a fatigue break point in the vicinity of 1/4 to 1/5 of the single cycle value.
And your point about loyalty to a company's position? When I am representing the company, I am careful about what I say, but I can explain where all this comes from:
http://www.barrystiretech.com/
Start with "Tire Standardizing Organizations", and follow that up with "Tire Load Tables". The load tables are published by these organizations and all the tire manufacturers follow those - in fact, their representatives sit on the committes that are the source for these tables. It is these tables that define the load vs inflation pressure conditions for the various types of tires.
It turns out that the burst pressure of a given tire is an artfact of what is needed to prevent a fatigue type failure. The company I work for has discontued bursting tires because it tells them nothing of real value. They have charts that specify the strength needed in a tire and are based on DECADES of experience and testing.
So I am NOT parroting a company position. I am showing you the sources of the engineering principles used to design tires.
And lastly, tires do more than just consume fuel. They provide grip and a comfortable ride. They are required to provide directional stability and long wear properties (some better than others!) ALL these things have to be considered when we talk about the affects of inflation pressure.
If there is a conspiracy going on, it's that the tire manufacturers are providing a source document for the vehicle engineers to use - and both groups have agreed to abide by those documents. The tire manufacturers are then free to design tires based on those documents and the vehicle manufacturers are then free to design vehicles basded on those documents.
If a vehicle manufacturer wants to change the way tires are designed, all they have to do is petition the tire standardizing organizations for a different set of standards - just like they've done in the past. The standards are constantly being revised to reflect the needs and desires of vehicle manufacturers. That's why the standards are published yearly - to provide updated information to anyone who wants it.
The current standard (and I'm referring to the passenger car tire load tables) have been in place since the late 1970's - over 40 years. It is quite possible to replace that standard with another one that takes advantage of the improved fuel economy that higher inflation pressures provide - BUT - that would result in a whole round of tire and vehicle redesign to accommodate that change. It's been done before and can be done again.
And to the point of burst pressure, any standard that increases the inflation pressure would also result in increased burst pressure - It's just they way the Physics works out!
