The new truks sit higher in the back due to the higher bed rails on the newer body style. Has nothing to do with 20" wheel option. Overall tire diamater is still the same whether you have 17, 18, or 20" wheels. 15" wheels aren't even an option with the size of hte brakes on the new trucks. Shoot I can't recall seeing a new truck offered with 15" wheels in well over 10yrs. Do they even still make 15" tires??? If you decide you don't like the 20" rims I am sure there are plenty over at the Duramax forum that would be willing to trade you their 18" rims.