It seems that a lot of motorists never think much about the space-saver spare tires fitted to many modern vehicles, and abuse them far beyond their design limits when they use them. I learned that when I went to a local tire dealer to replace my twelve-year-old donut tire, which was showing signs of oxidization as the rubber perished.
According to the dealer, any tire is supposed to be replaced after nine to ten years on the wheel, including donut spare tires. The problem is that so many spare tires are mounted underneath the vehicle, where they're hard to see, that they don't even get their pressure topped up, much less inspected for wear. People seem to forget they exist. The assistant dealing with my car said that he's had several incidents where vehicle owners had a flat and tried to put on their spare, only to find it was as flat as a pancake. To make matters worse, some of them then tried to repair their spare tires using cans of tire sealant refill, sold at many auto dealers and supermarkets. Trouble is, those cans are designed to pressurize full-size tires, not a donut spare with its much smaller internal volume; so they inflate the latter too much, which - given perished rubber or weak spots - can rupture again almost immediately.
Another problem is that regular car wheels are manufactured, reinforced and stressed to handle forces such as acceleration, cornering, minor bumps and potholes, etc. On the other hand, donut spare wheels are produced by simply stamping the wheel out of sheet metal. They're not designed for extreme use. The vehicle manufacturers warn buyers about that, recommending a maximum speed of 50 mph and maximum distance of not more than 50 miles. In other words, the donut spare is designed to get the vehicle to a place where it can be repaired, and no further. However, many drivers seem to regard them as interchangeable with the "main" wheels and tires, using them for days and weeks on end, and running at normal highway speeds. Inevitably, many donut spares that are abused in that fashion will fail on the wheel, and perhaps cause a serious accident in the process. Apparently some drivers have even tried to sue the donut spare manufacturer, on the grounds that the wheel and/or tire "failed" or "was defective". When the facts emerge, and it becomes clear that the spare was abused far beyond its recommended limits, the lawsuit inevitably fails, leaving the luckless motorist to pay the defendant's legal fees.
The final problem I learned about from the dealer was that some (not all) manufacturers may make a given model of vehicle for ten years or more. They'll order spare wheels and tires in bulk from a supplier, hundreds or thousands at a time, but donut spares are not ordered to a specific wheel and/or tire measurement. Instead, the overall dimensions of the spare will be specified. That means a larger wheel diameter with a smaller tire may fit those dimension, or a smaller wheel diameter with a larger tire. The only common factor will be the spacing and size of the holes for the lug nuts. It's up to the supplier to send an order that meets the overall dimensions. The next order, or one from a different supplier, may meet the same overall dimensions, but with different wheel and tire diameters, widths, etc. Therefore, when ordering a replacement donut spare tire, it's essential to note the measurement of one's existing tire and order an exact size match. Otherwise, a tire that may be used by some vehicles of that model might be supplied - but it may not fit your specific vehicle's spare wheel. Tricky!
As you can tell, I learned a lot. The dealer has ordered my new spare tire, and it'll be here within a couple of days. I'm looking forward to being a bit better equipped, for safety's sake.
Peter
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