I agree, air pressure has to be the same on each side for proper steering; especially the front.

After the first 10,000 miles on our S80, I found that the most even wear across each tire after rotating them every 2000 miles from new happened when the front tires had 34 ppsi and the rear ones at 27 ppsi. I know; that's not what the specs call for but here's what happens when these pressures are set.

Tire bulge is about equal comparing front to rear. The rear tires don't bulge quite as much with one or two people in front, but when there's four or five adults and some luggage in the back (max load) all tires bulge about the same. Tread wear across the entire footprint is very even. And the ride is excellent.

Putting 39 ppsi in all four resulted in a very hard ride and the center of the tread was wearing much more than the outside edge. In fact, about one-half inch of outside tread on the rear tires didn't even contact the road enough to put dust on them.

My Michelin dealer says the 34 front and 27 rear will give not only excellent tread wear and ride comfort, but about maximum safety, too, because maximum footprint is attained on each tire.

Go figure....