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So for the first time I've got separate winter and summer tyres for my car (I'm in a mild climate, so it's not usually necessary, but this one came with winters, so I picked up a set of summers to go with it)

What's the best way to store the ones I'm not currently using to ensure the longest life & avoid damage? They're mounted on rims, and I've currently got them lying flat in the back of the garage, so they're out of the light. Is there anything else I ought to do? Should I keep them at running pressure, or change it?

Well, for the winters, wrap in bags to keep the light off them, as UV degrades the compounds that make them work well in ice and snow. The back of the garage sounds fine, make sure they are not on a small stone or something as that will cause a puncture over time - I usually sort some small wood pallet or somesuch. I start storing mine at the correct pressure they run at then check them when re-fitting.

The summers I tend to treat the same but they don't have so much problem with UV...

Both summer and winter sets only do me two seasons then they get changed, and winters never do more than 2 seasons anyway as time, temperature, sunlight and use degrade the compounds.

This article from TireRack has some pretty good tips. To summarize the most important points:

• Keep the tires out of the sun to avoid UV rays


• Clean and dry the wheels/tires, and then store them in airtight (taped) plastic bags


• Keep them in a cool, dry location. Basements are usually better than garages as they tend to have less temperature and humidity variation


• Keep the tires away from sources of ozone, such as furnaces and other electric motors with contact brushes

Additionally, its especially important if you have high performance summer tires to keep them from getting too cold. These compounds are designed for higher heat environments and can degrade or even crack in very cold conditions. People living in cold climates should avoid storing them in unheated garages.

Most deterioration of tires ( other than wear) is oxidation , caused by - oxygen . So when normally pressured the partial pressure of oxygen is about three times the atmospheric oxygen pressure causing three times the oxidation. This factor is enhanced by higher temperatures. Oxidation is not a concern if inflated with nitrogen. So , if not inflated with nitrogen ; deflate and store in a cool place. Carbon black stops the UV. However , I don't think any action is worthwhile unless you are storing for more than several years. Long ago , at an Akron rubber testing laboratory , they put rubber test samples in an ozone (oxygen) chamber and heated it moderately ; It simulated aging of rubber but I forget the numbers ( it was about 1961 ).