I think I'd rather try to use the bottom of my mate Chris.
Dropping a tyre on someone is all very well, but if Chris drops one, then all hell breaks lose. My would be assailant would be paralysed instantaniously, whereas numerous away trips stuck in a car with Chris has trained my senses sufficiently that I would have a few vital seconds to finish off my choking and spluttering opponent before I collapse in a choking heap myself.
Hoof of Cow looks handy, but I'd be worried about it just being too short - you couldn't really get much power behind it if that's all there is - the only real option is bludgeoning, which is, I feel, over-rated.
If you could have two of them however, and mount them at either end of a STAFF, possibly featuring a rubberised central grip, you'd be nigh-on unstoppable. I'd accompany such a weapon with a cow-hide cloak, for purely aesthetic reasons of course.