All tires have roughly the same shelf life. How you keep it may stretch it out a little further, but even in a sealed trunk compartment, once you get much out past 10 years, you are taking a chance with any tire, including an unused spare. Especially if the spare is going to be working harder or carrying a borderline load on the road.
I would resist the urge for a fifth road wheel. Been there, done that. It gets really expensive (especially if you want matched wheels) and is a lot of unnecessary work, and will also soil up the stowage compartment eventually. Stick with a dedicated spare on a car. Drop it in, and leave it there (other than to check pressures).
Start with the premise that a modern spare is for emergency use, so it doesn’t need to top the Tire Rack survey or last for 70,000 miles. It only needs to get you to the next service station. Most today are irregular sized to the road set, and can cause drivetrain issues on some cars if left on for too long. It’s likely not ever traveling more than 15 miles, and probably will be rolling under 45 mph. That thinking can save money.
And most spares never get used. So whatever you spend, there’s a good chance that eventually you’ll be tossing an unused tire as it ages out. If you have an option, compact tires are considerably cheaper than the full sized ones. So you’ll only be tossing $100-125 vs. $200+ six, eight or ten years from now.
There is nothing wrong with a compact spare as long as you maintain it and don’t over use it. Otherwise, a full size spare is good if you want to finish off a trip before repairs, bearing in mind that many spare wheels today are not rated for the full duty life of a road wheel. For instance, a full sized Mercedes steel spare wheel is often only rated for 12,000 miles total, because they are not designed for more than that.
On a full size spare, I always buy the cheapest one that meets the size and load specs from a major maker. They are all warranted to last for 6 years, whether getting salt and gravel sprayed under a truck, or sealed away in a big German trunk. There is nothing wrong with dropping in a $175 Cooper all-season spare for your $300 Pilot Sports or LTXes.
If you are lucky enough to have the same road and spare sizes, you can also toss one of the old ones on the spare when you get a new set. As long as it doesn’t go out much more than ten years, most used ones still have another 15-20 miles in them. Sometimes, a mechanic buddy can even hunt a used one down for you.
Again, you won’t be taking the spare to Watkins Glen, or doing the US80 coast to coast tour on it. You’ll be taking the flattened one for service straight away.
And if you have no spare wheel package at all, there are also a couple vendors that sell complete compact spare packages custom fitted to your vehicle as a turnkey option.
I just went through this nonsense on four vehicles. Most people never crack open the compartment, and after 12-15 years it becomes an archaeological site in there. Useless and often dangerous when you finally need it on the side of the road.
But if you have just one road breakdown, you’ll be glad you invested in decent spares.
