my light came on for this, how do i know which tire it is? and what psi do i put it at? so i can just go to the gas station and put air in them.

You should keep a guage in the car, check the pressure on all tires and fill to 32psi (i think). The spec is on the driver's door jam. Some people like to run a couple of psi more, should also be OK. I run 33-34

I run 30psi all around for a smoother ride on 20's

carry a guage and check the tires on a regular base

The low pressure notifying system does not identify which tire is the culprit, and it could be more than one, so as noted above, check them all. Don't forget to check the spare now and then too. with the full size spare it is a royal pain because the valve is mounted down!!

A dial-type gauge is more accurate than a pencil gauge. I use the one from CarTalk.com. It hold a reading and has a flexible hose to make it easier to get the reading.

actually the gas station i went to you can set the psi and it will say stop when it reaches the psi you chose. its cool

+1.. I try to make it habit to check every time I fill up with gas... Or at least every other.

I'd check it immediately afterward with a good gauge the first few times to be sure their machine is right. We had those when I was a kid and I never knew if they were accurate.

On 20's I would do the same. Do you have any concerns on rim damage if you hit a pothole tho? I could never do 20's here in NY.