I've been quite delayed in getting back to this but I'm past the body shell and I'm starting on other details. I'm trying to get the tires sorted out now - a revolved NURBS shape with a displacement map. Having a few troubles however and would love advice if it's out there.

1. Just to the right of the little red dot I've added to the image is an unexplained discolouration. I'm rendering with MentalRay.

2. I understand it's better to uncheck Feature Displacement in the attribute editor of the object you're applying a displacement map to and to create a MentalRay displacement approximation. When I do this, the tire just ends up a little squashed and there's none of the details from the displacement map. When I check Feature Displacement, I get the attached image - not bad but a little edgy here and there.