Eeeeek this is one of those subjects which sound dead easy until you start doing the maths. I would imagine that what you're really after is Drawbar Pull....since you're trying to pull vehicles along.
Just as with HHF above I'm purely a layman, but I've sat in Traction classes and my eyes have glazed over just like everbody else's.
To see how lairy this can get, follow this link...
http://en.wikipedia.org/wiki/Horsepo...nt_definitions