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In all honesty, if you must store your car for a long period of time, especially outdoors, simply invest in a set of cheap used wheels and tires.
Take off the good ones, store those indoors and leave the cheapos on the car.
For long term storage, you could also jack the car up off the ground. Some people remove the tires and drop the air pressure to 10-15lbs for stoirage.
If you store the tires indoors, don't place them next to an electric motor. The ozone gases given off by a large motor (refrigerator or freezer or even a furnace fan motor will attack the rubber.
I know some Dodge Viper guys park their babies on carpet with some thick dense rubber from Lowe's. Does this help long term....? Sorry guys, but I only drive my C4 once or twice a year!