Could somebody explain the variables that come into play with regard to tire pressure? I notice that on the door jam there's a range of pressures for each tire and wondered what is optimum. From what I can tell, the following variables have an effect on the tire pressure that should be used, but if somebody could explain how we can adjust tire pressures to accomodate each situation to create the best handling and fuel economy, that would be great.

Weight in the car. Do you run higher or lower pressures with more passengers and cargo? I have a 330CiC and usually I'm the only one in the car, but I'd like to know how to adjust pressure if I carried a full load. Do I run toward the top of the recommended range if it's just me?
Temperature. How does this affect the tire pressure I should use? I live in an area of California where it doesn't get down to freezing temperatures often, but in the summer it is often 90-100 degrees Farenheight.
Road conditions. During the rainy season do I need to adjust my pressure? How about if I were to drive in the snow?Also, it's been mentioned that "cheap" tire gagues don't work well. I have an analog "pen" style gague (with the long piece that pops out when you check the pressure). If this is not an accurate type of gague, can somebody recommend one?

Weight in the car.

More weight, more pressure. It's the air that supports the weight of the car, passangers, etc, not the tires. The door sticker states this too.


Temperature.

As temperatures get higher, the tire pressure will go up with the temp. You'll have to bleed it down when weather gets warmer, and add pressure when weather gets colder. This is because, once you find "your tire pressure" (cold pressure, it's how we set it if we go by the door sticker states it), you want it to be the same.

Warmer weather, pressure gets up, you bring it down to the right pressure.

Colder weather, pressure drops, you get it up to the right pressure.



Road conditions.

Maybe not that high of a pressure for the rain, to make the car more predictable and forgiving.



Street tires have soft sidewalls, and tend to roll alot on corners. So they like higher pressures than the ones usually stated on the cars' stickers. Mine said about 30 psi front and rear, and after trying them all, I'm set at 36psi on the 4 tires. Sticker said 30 psi. The car is the one on my sig.