Which gas is present in car Tyres?

For normal driving, they use compressed air, for tires used in racing, nitrogen is commonly used because it doesn't have pressure fluctuations like regular air and helps keep the tire a little bit cooler. 

Nitrogen gas has been used to inflate the tyres of racing cars, aircraft, and heavy commercial vehicles for some time.

The same air you breathe with every snort.

100% what Robsteriak says.

B@locks to molecule size !

How is it possible that anyone does not know that tiyres or tires are filled with air and that air is oxygen and nitrogen?

21% Oxygen,  78% Nitrogen, 1% other gasses. Race tyres are inflated with pure nitrogen, but the benefits for a normal road going car outweigh the cost.

Air. It’s already 78% nitrogen anyway.

Racing cars need dry nitrogen filling because regular air contains water vapour, and as that gets hot under the extremes of racing it turns to steam. That in turn makes the tyre pressures higher than intended. It has nothing to do with the size of N2 molecules.

No normal road going car needs nitrogen filling, but it is a nice little earner for tyre shops, and owners have to just trust that they get what they paid for: unscrupulous shop owners have been known to simply refill empty N2 cylinders with air. 

And yes, in response to another answerer, I do mean “tyre”. Not everyone uses American English spellings. Both spellings are correct depending upon where you’re from.

Air. 78% Nitrogen and 22% Oxygen - give or take.

Car TIRES (not tyres) are filled with normal air. 

Normally we inflate car tyres using compressed air. Air is mostly nitrogen and oxygen. For high performance tyres, pure nitrogen is often used because its larger molecule size makes it less likely to leak.