I'm not sure you'll find a tutorial that's terribly specific to tractor tires, but Lisa's advice is pretty solid - just build a small section of the tread that will be repeated and replicate that.
There's a tutorial at
http://www.supercoldmilk.com/ac3dtut...woody_tire.htm that shows how to make a "normal" treaded tire using this method - you could possibly adapt this to build a tractor tire...
Good luck