[TheBear]

I can't remember my original source, but after a quick google, I've found lots of websites suggesting growing potatoes in tyres, and one suggesting there might be a problem with it.

positive link
not as positive

In fact, this site debunks the idea that it's bad:

There are mountains of old tires out there. Americans keep on rolling and tires keep on wearing out. Every year there is almost one scrap tire created for every man, woman, and child in the United States. In 2001 alone, Americans discarded nearly 281 million tires, weighing some 5.7 million tons. All of those old treads can provide a lot of good growing space, and we’re just the folks to put them to use.

There is no appreciable risk in using recycled tires in the vegetable garden. While it is a fact that rubber tires do contain minute amounts of certain heavy metals, the compounds are tightly bonded within the actual rubber compound and do not leach into the soil. One of the ingredients in the rubber recipe is zinc. Zinc, in fact, is an essential plant element. I also expect that rubber is safer to use than treated lumber that contains copper and arsenic. Tires are durable. The very qualities that make them an environmental headache make them perfect for our uses in the garden. Once they are in place, they won’t rot and will likely be there for your grandchildren to use.

Let’s take a look at some ways to recycle old tires and literally reap the benefits. Gardening with recycled tires has many benefits besides those directly with the garden itself. It puts to use an article that might otherwise end up in a landfill or other disposal site. Those of us who are into “growing our own” are often on the lookout for ways to increase production with a minimum of effort. Gardening with tires presents several good ways to do just that, while at the same time helping to recycle the old treads from our automobiles and other wheeled conveyances. Stop by your local service station, recycle center, or tire retailer and ask them to save some tires for you. Currently, dealers charge $2 or more to dispose of used tires. Since they charge the consumer to take the old tires and have to pay to have them disposed of, they will likely be happy to let you have all you want. Most tire centers will have a stack of old tires out back that they will give you permission to root through.