I suspect that you are right that changing the outside shape of the vehicle affects mileage. Each of the add-ons would undoubtedly decrease mileage, however so slighly. When you add up the effect of each gizmo, the net effect may be significant.
No driver is going to be able to tell the difference in mileage on a van with or without, say, mud flaps, but a wind tunnel and sophisticated equipment can definitely tell.
Officially reported mileage is very important in selling some cars, particularly minivans. 1 mpg really can mean a lot in terms of marketshare and price, hence the pressure to cut as much wind resistance as possible.
I think this is just one of the reasons such features as mudflaps and dooredge protectors are optional. The standard vehicle gets tested and rated with the least number of external options. What the consumer does after that doesn't affect the offical mileage rating.
IMHO, anyway...
Regards,
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Maugham
"I plan to live forever. So far, so good"
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