Is it okay to use old car tires in the garden?
There are a number of different views floating about and there seems to be no definitive answer to your question. The majority seem to be in favor of the use of tires as planters, hot beds, composting bins and insulating walls for greenhouses. There are quite a number of other projects you can create.
Some people are skeptical, as you are, that there are still toxins left in the old tires that might leach out into the soil and be absorbed by the plants. However, the negative reports focus mainly on the chipped rubber mulch and astroturf made from old tires.
There are quite a few folks who love their tire planting beds. There are downsides and upsides, depending on what area you live in. The black rubber tends to superheat the soil. One solution is to just paint them a light color, or you can use it to your advantage like a cold frame. Quite a few people have perfected the method of creating towers of tires for growing potatoes, creating a compost bin or growing tomatoes.
If you decide the risk of growing edibles is too great, there are a number of other uses. Save your old tires, or go out and rescue a couple from your local landfill, find some bright paint, and create some funky yard art. Then plant some colorful flowers in them!