I have actually inflated up to 50 psi when I first got them partly based on the advice given to me by the the "tire experts" where I bought them. I thought this was worth a try. I proceeded to let the tires deflate naturally (and remarkably all at the same speed) over months until it got to about 33. I could definately hear the whine on every turn and gas mileage getting worse at that point. The ride was confortable especially on rough roads as you might expect. I haven't do the chalk test (good idea though) but the tires do look like the contact area is the whole width of the tire at 40 psi.