Where does all the rubber worn away from tyres go?

I've just been doing a quick calculation. If a tyre lasts for about five years, I reckon it comes out at about 10 000 tonnes a year of rubber in the UK. Tyres are actually really bad polluters because they don't only contain rubber but also a lot of heavy metals too. That's why when people say let's just burn old tyres, it's really bad because the toxins go up into the atmosphere and drop out into the soil that animals eat. If we focus on the US, 300 million people live there. Let's assume that they all have two cars per family of four. A car has four wheels, which means that in the US at any given time there are probably roughly 600 million tyres in use in any given year. Let's assume that the tread on a rubber tyre is 10 centimetres wide, the circumference of the wheel is 3 metres, and the thickness of the tread is about a centimetre. That means that the volume of rubber on a wheel and rubbing out is about 3 litres. If there are 600 million tyres and you times that by 3 litres, and then convert that to metres cubed, that's a staggering 2 million metres cubed of rubber every single year just in America. The density of rubber is 1200 kilograms per metre cubed. That means that there are 2 billion kilograms of rubber in tyres in the US. If you assume that they last for four years, that means that roughly two billion kilos gets lost or worn out every four years. That's a staggering amount.