It's modeled with polys. I used the technique of making one section of tread, and then duplicating it in a long row, and finally adding a non-linear bend deformer to bend it 360 degrees. You can find a tutorial on that technique here:

https://www.danielbuck.net/

It's expensive with the number of polys, but gives you a lot more accuracy on the tread. Displacement maps can work well, but for me it takes just as long to make a nice displacement map as it does to model it directly.