Draw a chalk line across the tread of the tires. Drive through a couple of revolutions of the tires (Up and down the driveway or some other smooth surface). See how much and what portion(s) of the chalk line have dissappeared. From this, you can get a feel for the best tire pressure to run your tires at. If the chalk line wears off at edges but not in the middle, need to run more air pressure. If the chalk line wears in the middle but not the edges, too much air pressure.