Tires are pretty durable, but look on the side of any busy freeway and you’ll see a car or two jacked up with a flat tire, the wheel hub and bearing assembly exposed for all to see. This happens often enough that car manufacturers include a spare tire to get you to the nearest tire station to purchase a new tire. There are even manufacturers warnings about how many miles you can drive on a spare, but we’ve all seen people who have left a spare on for longer than they should. What harm can this do and how is a spare tire different than a regular tire?
Spare tires are typically more compact than a regular tire to save room in the trunk. If you had a full-sized spare most smaller vehicles would have no trunk space left, so car manufacturers make them smaller and narrower than regular tires to fit in small spaces. Spare tires are also lighter than regular tires because they are made with less reinforcing materials like steel and polyester. This means spare tires are not as durable and have a greater chance of being punctured or damaged by debris in the road.
The reduced durability and small size also take a heavy toll on the vehicle’s handling. A spare can pop from slamming on the breaks, and it would be very difficult to maneuver around debris on the road with a spare on one tire.
The greatest issue with leaving a spare on your vehicle is the wear on your differential. A differential transmits engine power to the wheels from the transmission, but it also enables your left and right tires to move at different speeds. This is important so the car can turn corners – when turning, the inside tires rotate slower than the outside tires. When driving straight, the differential is largely unused in this capacity and is not worn down. However, when you have a smaller spare on one tire, it must turn much faster than a full sized tire to keep up. This means the differential must work constantly to allow the tires to rotate at different speeds, as if you were constantly turning a corner. This puts a lot of stress and wear on the differential that is abnormal and can cause the grease lubricating the differential to wear away.
Getting a new tire is much less expensive than replacing your differential, so it’s always best to get to a tire store right away to secure a new tire. There are many tire shops that will put a warranty of up to 60,000 miles on their tires, so if one goes flat within the warranty period they will replace it for free. So next time, if you get a flat and have a spare on your car, don’t delay in replacing it with a new tire.