The idea is not bad. There might be one catch though. Tires age and itīs especially bad when they are not in use. So it would be prudent to watch for the age of the tire too. At least the used tire should be replaced every time you buy new tires. Otherwise you might end up with an unuseable spare. After the first exchange you would have a spare thatīs not worn out.
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If it isnīt broken, it doesnīt have enough features yet.