It's a perennial issue with extra sets of tires and/or wheels: What's the best way to store them when they are not being used? Many people store tires improperly, and this can often shorten the life of your tires.
The essential problem here is outgassing: as the rubber ages it loses volatile oils through the outer layers of the tire. Normally, the flexing motion of the tire tends to keep the oils evenly distributed throughout the rubber, so that outgassing is a minor issue. But when tires are stored for long periods of time without that flexing motion, outgassing must be kept to a minimum to avoid drying out the outer layers of rubber to the point that they begin to crack instead of flexing. We call this pattern of cracking rubber that develops over time “dry-rot” and it's a sign of approaching doom for your tires. Here are the best ways to avoid it if you are storing your tires or even a whole car over a season or longer.
Get the Weight Off
If you're storing a car for any period of time, it's best to put it on jack stands and take the wheels off to store separately.
Keeping the weight of the car on just one area of the tires can help to flatspot the tires and the constant flex on only one portion of the tire can prematurely age the rubber. There are a few different gadgets out there which are supposed to help avoid flatspots, such as curved plastic tire rests that you drive the car onto, but these are extremely expensive and do not work nearly as well as good old inexpensive jack stands.
Clean Them Up
When you take tires or wheels off the car, this is often the best time to clean them,as you will have much easier access to areas that are less accessible with the wheels on the car. Tires can simply be cleaned with mild soap and water if necessary. Wheels can usually also be cleaned with mild soap and water, or with a non-corrosive, non-acid wheel cleaner such as Auto Magic MAGnificence, P21S or the like. Do not use any cleaner that instructs you to remove the cleaner within minutes, as this is usually an acid-based product. Do not use chemicals like tire shine or tire dressings before storing your tires. Make sure that wheels and tires are cool to the touch before cleaning, and make sure that they are completely dry before storing.
Tag and Bag
Mark the tires with the position they were removed from – I suggest using LF, RF, LR, RR on the inboard sidewalls – so you can replace or rotate them in the proper positions next season. I use a Markal B Paintstick to write on tires and rims.
Put the tires in large plastic garbage bags and try to remove as much air as possible before sealing them with tape. If the tires are wet or snowy let them dry completely before bagging them, in order to keep moisture out of the bags as much as possible. If you really want to completely prevent outgassing, there are storage bags with valves that can be hooked up to a vacuum cleaner to provide a near-airless environment for each tire. It's probably overkill, but relatively inexpensive overkill can still be fun.
Store In a Cool Dry Place
Tire rubber is built to soak up heat, and black rubber left outside will soak up solar heat remarkably quickly. Tire rubber is also built to dissipate heat quickly, but with heat dissipation comes high levels of outgassing that will quickly dry out the rubber. Store tires out of the sunlight, preferably in an area like a basement that is climate-controlled and moisture free. Garage or outdoor storage should have as little temperature changes and/or extremes as possible.
Whitewall to Whitewall
If you have whitewall tires or white-lettered tires, you should definitely bag them to avoid discoloring the white parts. But if you can't bag them or if you're just stacking them up to get bagged, stack them whitewall to whitewall. The rubber on the whitewall side is treated to avoid staining the white parts. The rubber on the back sidewall is not.