My only suggestion would be that the front grill should have a slight point to it, like a train's cow catcher. The way it's positioned now it would knock down the zed-heads and force them underneath. That's not a bad thing if you know the wheels will pop their noggins like rotten cantaloupes...but what if one gets hung up? It would really suck to step out after a day's work putting down the walking plague, only to have one grab you and chomp on your Achilles tendon.
Perhaps if only the bottom half of the grill protruded... Then it would be like a cow catcher and dozer blade combined and it would force the biters "down and out" giving you a better chance to run over the head.