According to this page, the majority of cars are running on tires that are only filled to 80% of capacity. What does this mean to the Everyman?
Not only are under(or over) inflated tires a safety hazard, but they also are a hazard to your wallet! According to the referenced study (from the site link above), the typical 12,000 mile per year driver uses an extra 144 gallons each year due to under-inflated tires. 144 x $4.00 = more than $500 savings per year.
The beauty of this tip? It's free! While a few places may charge you to use their air tank (and if they do, you're bound to find enough change underneath your driver seat), the majority (at least here in Minnesota) offer compressed air for free.
In order to find out how much air you need to put into your tires, you'll need to locate the manufacturer's recommendations, which can be found on the outside of the tire. While it may be hard to see, you should find it in raised lettering outside of the hubcap area of your tire. The tire should indicate the recommended PSI, which is the pounds (of air) per square inch.
You'll also need to pick up a tire pressure gauge, which will tell you what the pressure is currently at in the tire (and also allow you to monitor your progress at increments while filling your tires to their correct PSI). This gauge shouldn't cost you more than a few bucks (I even found a digital one at Target a few months ago for about $10).
This simple way to save some money should provide you immediate results at a low (if not zero) cost. Go ahead and give it a try! (Hey, I can think of ways to spend $500 per year - on things other than gas!).