This site has all sorts of timely tips:
By and large, properly inflated tires make about a 3% difference in gas mileage (depends a lot on how misinflated they were to begin with). Hey, I'll take a free three percent that'll also make my car perform better and my tires last longer! *
Surveys show that lots of people are driving around on one or more misinflated (usually that means underinflated) tire.
The best definition of "properly" inflated for all-around roadgoing duty is probably found on the sticker the manufacturer put on the door jamb or in the owner's manual.**
You really can't tell by eyeball on modern radials unless they are already pretty far off. Slap a gauge on 'em every month or every other fillup or something like that (no need to get neurotic about it unless you know that one has a slow leak) and know the truth.
Cheers, --Joe
* Significantly underinflated tires run hotter on the highway, and if memory serves, was widely thought to be one of the factors in the rash of blowouts with certain SUV tires a few years back.
** SUV and pickup people sometimes air them up higher (there's a max cold pressure stated on the sidewall) for carrying a heavy load, or air down for traction in soupy offroad conditions, but they too are best served for normal duty by the car maker's recommendations. Performance drivers sometimes use an old-fashioned technique called "chalking" or a new-fashioned one involving pyrometers to determine their best pressure, but not only are those advanced techniques, but they're really meant for sorting out the car for best handling, not for maximizing gas mileage.
