Underinflated tires create drag and can decrease gas mileage by 3 percent or more. It's simple to check your car's tire pressure using an inexpensive tire gauge. Most tires should be at around 30 to 35 pounds per square inch (psi), and your owner's manual will tell you what psi is ideal for your car. You should check your tire pressure at least once a month and add air when necessary.
The trick with tires is to maintain a safe tire pressure without overinflating. While slightly overinflating your tires can increase gas mileage, it also ups your risk of blowing a tire and makes it harder to brake and turn the car. You don't want to put fuel savings ahead of your own safety.
Maintaining your car can go a long way in getting better mileage, but for some drivers, switching to a fuel-efficient vehicle is worth the money.