For every two PSI that all of your tires are below the recommended level, you lose 1% on your gas mileage. Most car tires are five to up to 10 PSI below the normal level, so that means by just airing up your tires, you can improve your gas mileage by up to 5%. It’s easy, too. Just read your car’s manual to see what the recommended tire pressure is, then head to the gas station. Check your tires, then use the pump to fill them up to where they should be and save!