The number of cars in the entire world surpassed the one billion mark, and normally they have four wheels. Changing the tires is usual, several times in the vehicle life cycle. What does it happen with this insane quantity of tires? They are made for not being fully biodegradable, that's clear.
Some could draw a smile in his face when they know that the old tires are being recycled, actually reaching 80% in several countries. The bad news is that the recycle operation could bring with more pollution contributing to the greenhouse effect due to increasing carbon dioxide in the atmosphere.
Some are also used as fuel in cement kilns, others for construction purposes. Even there was who used and old tire for renewing his shoes, cutting a part as sole and sticking it. Also who started a shoe business with this source as primary material.
Someone burn them to get heat, but that's extremely bad for the environment since it creates a toxic cloud that increases risks toward several health issues, some of them severe. And it's not the first time that stockpiles of old tires were accidentally burnt, millions of them. The environmental impact was very noxious, and for solving it was necessary to invest a lot of work and other resources.
Sometimes the best aspect when you analyze this topics is to remember that there are other transportation methods that are especially interesting because they increase health and avoid pollution. Walking is also a good exercise.