I've been the car business for over 30 years. My thoughts on tire pressure are based on experience, as well as tire and vehicle mfr's info. On customers' tires I start with the mfr. recommended pressure and adjust for weather when necessary. For example in fall I'll overinflate by 2 psi to allow for the loss of pressure due to upcoming colder temps. On my own vehicles that I'm over, under and around all the time I'll check tread depth at the outer, inner and center grooves and let the tires tell me what they want. Ideally, the correct pressure will allow the tires to wear evenly, as GhOsT 1321 says in post #6. Typically on a FWD car, I'll be 2Lbs over sticker in front and sticker pressure in the rear, but I've been as high as 5 lbs over and as low as 2 lbs under, depending on the vehicle and driver. I also rotate every 5K miles at the most, sometimes side-to-side, sometimes front-to-rear, sometimes "X" pattern. As you might expect, with a little extra OCD care, my own tires last a long, long time.