jarednjames said:
Under inflation can cause a number of problems. It reduces the performance of the tire for one and can prove to be rather dangerous. Not something you can shrug off.
Incorrect alignment may not immediately present a problem, but it will increase wear for a start.
Also the abnormal wear will tend to be asymmetrical, and that will take the tires out of their engineered optimum performance zone. Not good, especially in demanding conditions. Tires are a whole lot more important than most drivers realize.
The next-to-last operator of what used to be my favorite tire store was a stickler for safety. If you bought tires from him, he'd inspect them, check inflation pressures, rotate them, etc, all for free as needed. As you changed from winter to summer tires and back, he'd do that for just a minimal charge for mounting and balancing. He knew how to generate customer loyalty.
Once, before a very demanding back-road trek to the Maine north woods on roads that were notorious for being slatey, and slashing tires, I stopped into see if I could get an extra spare tire for my Pathfinder. Artie looked at my Pathfinder's tires, selected a suitable rim from his pile of cast-offs, and fitted it with a lightly-worn used tire that closely matched the characteristics of my mounted tires. I reached for my wallet, and he said "no charge" "have a great fishing trip". Part of the charm of living in small towns where everybody knows everybody else. Believe me, I sent a LOT of business his way.
He had a good eye for abnormal wear characteristics, too. It was a good idea to stop in once in a while and shoot the breeze and play fetch with his lab-Rhodie mix. He'd walk around the vehicle while we were talking and say something like "I see some cupping starting. You might have a toe-in problem." and he was always on the money.
Edit: Zom leap-frogged me. Asymmetrical wear can be a lot more dangerous than running on worn tires with less tread.