If your tire is under-inflated by as little as 6 psi:

• Uneven tire wear
• 5% reduction in MPGs
• Tire wear is accelerated by as much as 25%
• Tire failure or Blow out

Tire pressure rises or lowers itself by 1Psi for every 10 degrees Fahrenheit.

You Should always set your tire pressure in the morning so they will not lose pressure as the day heats up. Some days the temperature can easily change up to 50 degrees and then you can lose as much as 5psi of air pressure…

The best place to find the proper inflation is in the owners manual. NOT on what is written on the sidewall of the tire that is just the min and max of what the tire can be set at.

For information on tread depth Go Here and for more information on tires and the source of the data used in this article i recommend reading some info on Tire Rack

Honestly the best place to get car info on the net is a car forum. These communities of motor heads share their wealth of knowledge for free. It can be intimidating trying to find the answer to your question. If you use some basic forum searching techniques and etiquette can go along way in helping you get the most out of your make and models forum.

1. First before asking any questions SEARCH. Your question probably has already been asked.Check the FAQ section and use the search feature on the forum.      (Some vbulletin forums by default list the results not by relevancy but by most recent post date. Simply go to the advanced search screen and check the box that says sort by relevancy it may help you find your answer. )
2. Ask your question. First be sure to post in the correct section. If there is a newbie section post in there. Be sure to give all the details of your car. Make model year stick or auto etc…  Mention that you searched for the question and thank them for their time.
3. When you get answers tend to listen to the ones from users who have higher post counts. They usaully are more reliable source of information.