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Space saver spare tires are smaller wheels and tires that can be fitted in the event of a puncture. They are generally only rated to 50 mph, and short distances. This Quora answer sums up the impression I get about their performance characteristics:

Q: Why do spare tires have speed limits?

A: Because ALL tires have speed limits, the difference is the particular “emergency use only” tire has a significantly reduced capacity for load and heat dissipation and so the “limits” are “prominently displayed” so you don’t exceed them, destroy the tire and wreck your car”.

Why are they built this way? It is certainly possible to build smaller tires that can take the load and heat dissipation of high speed, motorcycle tires and old F1 cars are the obvious examples.

Having a different size wheel is obviously going to affect handling, but is there a mechanical reason for a space saver wheel/tire to be less able to handle the load and heat dissipation of 70 mph than a regular wheel/tire?

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Why are they built this way?

They are built this way for the usual reasons:

  1. To save money.
  2. To save space.
  3. To save weight.

These tires are not built the way normal tires are built. They have fewer plies, less rubber compound, thinner tread depth. Those things make them lighter and less expensive to manufacture. They also make them smaller so that they fit into a tighter space in the vehicle. This puts additional strain on the differential and steering components and those stresses are greater at greater speeds. So limiting the speed helps keep the vehicle safer.

Yes, these things are a compromise in every sense of the word.

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