<VV> Temp Tire Myths. Now tire safety.

roboman91324 at aol.com roboman91324 at aol.com
Sun Sep 1 15:59:49 EDT 2019

One last comment.  (Well, maybe not.)
I don't want to imply that mini spares don't have a useful place on our cars.  The fact that they are smaller is why they were introduced in modern cars and they offer the same benefits in our cars.  However, we need to be aware of their limitations just as modern car owners need to be aware.
1.  When introduced and forever after, compact spares have been accompanied by warnings on the tire, in the trunk and in the owner's manual.  These warnings are for any use of the minis, not just for use in the original car it came with.
2.  Any time you use a tire of a different diameter, you introduce a change that violates parameters that were intended by the car designers.  Mismatched tires have always been a concern but replacing the tires at all four corners with smaller tires are a concern as well.  This concern includes changes in diameter as well as mixing bias ply tires with radial tires, etc.
3.  Many don't realize it but all tires have the equivalent of expiration dates you see on food, etc.  Even if the tires have plenty of tread and look good generally, there may be issues you can't see.  This applies to compact spares as well.  Tires deteriorate quicker in sunlight but they all deteriorate, even in the trunk.  If you mount any tire in the engine compartment, expect accelerated deterioration from the heat cycling.
4. We all suffer from this next issue; myself included.  We tend to want to save money by using the best of the old tires we just replaced as a spare.  That tire can reside in our trunk for decades.  If we are diligent, we check it for pressure periodically but usually that is all we do.  The best way to use your tires is to rotate all five periodically so that they wear out evenly; then replace them all as needed.  Yes, the cost of five tires is 25% higher than four tires but the tires will last 25% longer in use.  Here is the simplified math ... Lets say that a tire has a useful life of 80,000 miles.  If you have the same four tires on the road continuously, you will need to replace all four when the car's odometer registers 80,000 miles.  However, if you rotate five tires, you will need to replace all five when the odometer registers 100,000 miles.  Each of the five tires will have been on the road with 80,000 on them because each will have spent 20,000 miles worth of time in the trunk.  Some tire shops will rotate your tires for free as a good will investment.  The thought process is that if they service your needs when you don't need to buy a tire, you wlll return to them when you do need tires.
5.  Based on this thread, I realized that the tires on my car trailer are very, very old.  They still hold air and have about 90% of their original tread but I wouldn't trust them at any speed.  The spare has never been on the trailer and is pristine.  I will replace all five.  I will probably slash the sidewalls of the old tires so that someone doesn't scavenge them thinking they have perfectly good tires.
Call me a safety nerd.

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