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Why Is a Car’s Spare Tire Smaller than a Normal Tire?

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Why is the spare tire noticeably smaller than a standard wheel?
Photo: robmba via cc/Flickr

You’re driving your car along the highway and you hear that sickening POP as that rear tire blows. You pull your vehicle over to the side of the road and go to obtain the spare tire from inside the trunk. When you lift it out, you notice that the spare is noticeably smaller and different in appearance than the other tires. Why is that? Why not make it a purely identical replacement to the four standard wheels?

If you’ve ever wondered these questions when you installed a spare tire on a car, we have the answer for you.


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The Reason Spare Tires Are Different

While some vehicles do offer a full-size replacement (fifth) tire or just a tire patch kit, the majority of vehicles include a compact spare tire also referred to as a “donut.” These emergency substitutes have a smaller diameter, narrower width, and shallower tread.

Spare tires are

  • designed to take up less space than a normal tire would when stored.
  • made to minimize the extra weight it puts on the car.
  • composed of a light-weight metal for its rim that makes it easier to lift and install.
  • not intended for long-term use like standard tires.
  • are intended to travel less than 100 miles and under 50 mph.
  • held to a different psi level.
  • created specifically for the vehicle it comes with.
  • cheaper for OEMs to make and include with the vehicle.

It’s important not to drive on a spare tire for an extended period of time, as the difference in size will gradually throw off the vehicle’s alignment and handling. In addition, their smaller circumference leads to a difference in RPMs compared to the other three tires. Because of its differences, compact emergency tires should not be attached to a drivetrain axle.

Spare tires have been around for over a hundred years, but we’re entering a new era. As technology continues to advance and alternatives like tubeless tire design eliminate the need for spare tires, these “fifth wheels” might soon become a thing of the past.


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Sources: Tire Rack, Your Mechanic