Informed drivers know that you can improve gas mileage by about 3.3 percent with properly inflated tires. Those in the know understand that in the summer, tire pressure goes up when a tire gets hot, and you should never trust a tire gauge reading after you have been driving around. Let the tires cool for a few hours before getting a reading. In the winter, of course, the inverse goes into effect. Cold weather will lower your pressure. Just letting your tires sit unused you will see a reduction in pressure month to month. This will affect your gas mileage in the end, unless you check your tire pressure regularly and keep tires inflated to their proper psi. Fuel efficient tires are not exempt from any of this-the same conditions can reduce your air pressure and gas mileage even with improved rolling resistance from fuel efficient tires.