Which is stronger, PET or PVC?
You need to define what you mean by "stronger".  Are you talking about resisting deflection (and thus rolling losses)?  Yield?  Break?  Impact?  Abrasion?  Lifespan?  Ability to withstand heating from its deflection as the vehicle rolls?  Also, any material can "withstand the weight of a car" if you use enough of it, do you know what sort of geometry you're talking about?

Lastly, the material properties of polymers depend a lot on the manufacturing processes and the details of the raw materials (chain lengths, branching, monomer ratios, etc) and additives.  A good example is PET.  Check out the difference in tensile strength between simple thermoformed PET and BoPET (Mylar), for example; they're vastly higher for BoPET.  And the reason is because it's elongated to near breaking at lower temperatures - thus orienting the polymer - and then given a heat treatment to relax it into its new shape.  

I think you need to define your problem statement better.  Think about what properties you want, your geometry, and how you plan to manufacture it.  Also, what are the parameters you're trying to optimize?