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I guess I'm not really sure where the questions are. If you are talking about the 10% figure, that is what I heard (read) in news reports from the tire makers during the Ford / Firestone lawsuits. I'm sorry they decided not to include those figures in their general FAQs, but I DO distinctly remember Firestone claiming that they had recommended a minimum pressure for the tires they were supplying to Ford (not more than 10% below the sidewall max), and Ford (in it's infinite wisdom greater than the DESIGNER of the product being used) printed a lower number on the door stickers. Stupid customers then didn't bother to maintain even THAT pressure, and the tires did what they do: lost about 1psi / month of operation, dropping FAR below what Firestone had specified.
Does that mean the tires would magically not fail, since the all powerful FORD had decided to "suggest" to the owners a lower pressure than Firestone said? of course not. They failed, peeled off, and people died.
Ford did much the same thing with the Navistar diesel engines from International. International provided the engines with specific maximum RPM and HP guidelines to prevent damage to the engines. Ford (again with the infinite wisdom stuff) tossed the ECMs provided by International, and specced out their own ECM maps with much faster RPMs for normal operations, and higher boost levels to reach more HP at those faster RPM numbers. They did this INSTEAD of installing a transmission with more and lower gearing... Which would have cost more money than swapping a computer out.
The engines started failing right and left, as I'm sure you have heard about in the news.
The point is that the supplier of a specific component has designed that component to work in a variety of conditions, and has specced it out at a certain level according to their design. The OEM is taking that product, and then re-engineering the specs, without complete knowledge of the possible results. The tire specs provided by a CAR maker (not a tire maker) are primarily designed to offer a soft cushy ride to the driver, at the expense of fuel mileage and tire lifespan.
Why would a manufacturer do this? B/c then they DON'T need to put in softer springs ($$) softer shocks ($$) or better engineer the design of the suspension ($$$$). Tires are a wear item, and don't cost the mfgr ANYTHING, as they don't warranty them for any specific lifespan. Ditto for fuel mileage. They would rather save 5 CENTS by not installing a drain plug in a transmission pan, and make the life of a technician that much worse... Because it saves 5 cents per vehicle. If airing-down the tires saves money so they can get away with using cheaper components elsewhere and let the tires absorb the shocks... Think about it.
Sorry for the book, but that sometimes needs to be said, so people think about WHY things are the way they are. Never assume business will "do the right thing" EVER without being forced. Seatbelts used to be an optional feature until the government FORCED the car makers to include them in every car.
On the tire pressure while unloaded, and the wear... I run full sidewall max in all my vehicles. In some cases (the RVs especially) they have tires that are MUCH higher load ranges than what is sitting on them. I have not had one single tire wear in the center, EVER. To me, it's all about controlling the contact patch. The contact patch may not substantially increase to the sides of the tread direction unless the tire is SERIOUSLY underpressured. But if the tire is partially low, the length will increase. With that length comes extra flexing as the round surface must flatten out and then re-curve continually while rotating, AND it is also flexing the shoulders outward as well. Think about those cheese-and-cracker packages we all had as kids. The ones with the red plastic spreader. Did you ever bend them in half to make a warm spot? That's EXACTLY what the tire is doing ALL the time. Modern casings of tires are much stiffer than even just a few years ago. With steel sidewall belts, tire mis-shaping from over inflation is becoming a thing of the past. What happens when the vehicle is unloaded compared to the carrying capacity of that fully-pressured tire? The contact patch will shorten along the tread direction, reducing friction. The steel belts maintain the shape, and the square surface to the road. Much older tires didn't even have steel belts on the tread, and certainly WOULD "balloon" into a rounded shape when lightly loaded or over-pressured. Modern tires don't.
All of this is based on my own experiences with all of the vehicles my family owns, since I am the one that maintains them. As I said, I run them ALL at full sidewall max, and not only do they NOT get center-wear, but I get the FULL mileage out of the tires, even while not rotating them out of position on the car. Your mileage may vary, but this is my story.
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) and if they fit with even with a little rubbing I will be getting me some.
