Fact is under-inflated tires will have an effect on fuel economy. A negative effect. So if your tires aren't properly inflated always you'll not only be spending more money on gas but you'll need to buy tires more often because they'll wear out faster. It is recommended that you check your tires air pressure each time you are at the gas station but many motorists I know don't. So back to the question: how often do you check your tire pressure?