Sorry for my poor English. Just got confused with a term.

In another post I wrote "tyre thread" (talking about the part of the tire that comes in contact with the road surface), and someone suggested that it should be "tread". I googled and found that they are both used by people, but "tread" is far more popular.

Is "thread" a correct spelling ? If it is, do the two words refer to exactly the same thing ?

Tread for the surface of the tyre and thread will be the steel bands inside.

thread is also known as string

See also: twine.

Tread is the correct term for the road contact surface of a tyre.
This holds true whether great chunky earthmover tyres or smooth slicks for racing.
Speaking as a one time employee of "Banglop" erm sorry, Dunlop, the formal term for the construction of the "bands" or "plies", whether steel or poly is chords.
Don't trust all google tells you lol.

Scawen also uses 'tread' in his tyre physics simulation article.

Tread is often used when referring to grippy surfaces. Thus, a car tyre has tread (except slicks, which just have a surface!), and most shoes have a tread.

Thread refers to string, twine, the parts of a rope, or possibly (though very rarely) the steel cord used in tyres or throttle cables etc.

Quote from tristancliffe :

Thus, a car tyre

Or tire, depending on if you're feeling American or not

Quote from Cornys :

Or tire, depending on if you're feeling American or not

Just noticed that I used both spelling in the same post. Oo0o0ops...