Naw, I dont think the tire complexity matters if your using ph_sphere(correct me if i'm wrong). I always thought that the physics engine applied the same sphere collision box regardless of your tires complexion. If your using ph_poly on the other hand(which I wouldn't reccomend) then yes you'd need quite rounded tires... but that is such a waste of polygons!

What you should really do is take your chassis/wheels and make them invisible. Then, create a new chassis and have it always follow the x/y/z and pan/tilt/roll of the one that is hidden. This new chassis shouldn't be enabled in the physics engine. The chassis should also have its wheels connected to it and each wheel should have a bone. Animate its wheels through the bone joints. What this will allow you to do is have the old chassis's tires at whatever size you want ect. without it being visible to the player. Provides alot of freedom in getting that "perfect" physics simulation that you want without sacrificing your chassis/wheel design(Cause tractor size tires that feel right don't neccesarly look right). You can also do scripted spinouts, flips, rollovers and suspension with this design. It's the best of both the scripting and physics worlds combined...