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Based off your numbers, it appears you are quoting figures off automobile catalogue, or a similar database, which the majority of are not measured but parametrically reverse engineered (shown as “estimated”). GM claims a measured CD of 0.45 for a generic GMT400 pickup truck, which while is not my exact configuration is the only measured CD I am aware of in open literature for a GMT400 platform. I personally trust this number more, but will agree to disagree on this point.
The transition from the GMT400 platform to the GMT800 illustrates the transition from detail optimization to shape optimization, which resulted in the loss of many very good aerodynamic details (flush wheel covers, flush/tight composite headlight to grill, flush door handles, etcetera) but overall a better tapered body shape.
There were not enough tweaks on the GMT800 platform over its 8 year run to cut the drag coefficient by over 40%, a 1999 GMT800 Silverado very closely resembles a 2007 GMT800 Silverado Classic aerodynamically. Many of the changes were for looks, not lowering drag. (e.g. use of the early 2500HD upper hood profile on later half tons).
Regardless, the stock drag coefficient will mean little after I cut and taper the rear of the vehicle.
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