Tire pressure? Affects to gas mileage?

Hi,
Kibbutz gave you an excellent answer (credited him with 2 thumbs up).  I would like to add 3 comments:
1.  If you have too low pressure in your
     tyres that means more steady (more
     contact with the ground), more fuel
     consumption (more friction), easier
     to slow / stop the car (safer in case 
     of emergency) and more wear, the
     car is slower / lower in it's
     performance.  
2.  If you have too high pressure in the 
     tyres that means: less steady (less
     contact with the ground), less fuel
     consumption (less friction), harder
     to stop / slow the car, and less  
     wear (in fact more wear per the area
     in contact), and better general
     performance. 
3.  The optimum is pressure that is
     recommended by the producer.
Petrol consumption is not the only parameter to consider.... safety is at least as important. 
Best regards,

You should check the tire pressure frequently, I check mine on a month to month basis, when they are a few pounds below, or a few pounds over, my gas mileage is affected.  With gas prices being so high these days, also when you drive don't stomp on the gas pedal, that makes you use more gas than you normally would.  Another thing, you should make sure you vehicle is properly tuned, ie plugs, wires, air filter.  Another thing you could try, and I do this myself every other month, is an intake flush.  If you have any friends that are mechanics, have them do it, its much cheaper than taking it to the shop where they can charge you anywhere from 50 to 100 dollars for the job.  All it takes is a can of carb and choke cleaner, sprayed into the intake, let it sit, so it cleans up the heads and the intake, and then start it up, it will have difficulty starting, but run it, rev it, and it should blow out alot of the carbon build up in your intake, that usually helps keep the intake clean, and more efficient on gas