The recommended tire pressure printed on the tire's sidewall is a guideline, and I suspect it's also to reduce liability. You can usually go below the minimum, and sometimes you may want to go above the maximum. Use that chart trakhak provided. Bear in mind that all tires seep air to various degrees, so after a while, your tires will be underinflated. So the ideal pressure for riding is not necessarily the ideal pressure when it's time to inflate. If you are not going to measure and fill up every week, it's a good idea to overinflate a little (5 to 10 psi) so you can forget. Everyone forgets.