In the past, if a tire got punctured, it would almost immediately go flat, which was the reason for the spare tire -- you'd always have the extra tire to replace the flat right there on the spot. But a spare tire is heavy, which weighs down the car and decreases the fuel efficiency. Tire manufacturers like Bridgestone are creating more run-flat tires that will continue to work up to 50 miles after your tire gets punctured. This gives you time to drive to a dealer and get the tire replaced. This new tire technology eliminates the need for a spare tire and can improve your car's overall fuel efficiency.