I would strongly caution against lumping all C load range tires or all D load range tires together. That is way too broad a brush. There are HUGE differences between different tires within the same maximum load capacity. I have used great and lousy tires in both load ranges.  

See http://www.busdepot.com/details/tires/ for our article on choosing a Vanagon tire.

- Ron Salmon
  The Bus Depot, Inc.
  www.busdepot.com
  (215)  234-VWVW

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