NOT a good idea. Over-inflated tires might increase fuel economy slightly, but not to the point where you'd notice the difference at the gas pump. Meanwhile, over-inflated tires will wear out quicker (in the middle), causing you to have to replace all four tires much sooner than would otherwise be necessary. This is especially true in any state that has mandatory yearly inspections. One of the things checked is tread depth. If you have plenty of tread on the outside but the tire is bald in the middle, it's a safety inspection failure, and MANDATORY tire replacement.
But to answer the OP question . . . improperly inflated tires can dramatically decrease fuel economy. If a tire is just a few PSI too low, your gas mileage might drop 10% or more. Thus, it's possible that a tire that looks OK (by visual inspection) could be costing you money buy wasting fuel.
Try to check your tires at least once a month, and inflate all four of them to the pressure recommended by the manufacturer of THE CAR. You will often find this information on a sticker inside the driver's door somewhere. If not, it will be in the car's owner's manual.
Gas stations used to offer free air to their customers, but those days are pretty much over. Get yourself a good tire guage for about 5 bucks and a pump of some type. Home depot (or was it lowes?) sells a tire inflator that plugs into the cigarette lighter socket and only costs about 20 bucks. Good to keep in the trunk for emergency use, anyway. -Dave
