Posting this link because it addresses the age-old search query: will I get better fuel economy with my truck's tailgate up, down, or removed entirely?
It's also one of the better/most comprehensive studies I'm aware of.
Pickup Truck Aerodynamics – Keep Your Tailgate Up
http://ecow.engr.wisc.edu/cgi-bin/ge...hi1/pickup.pdf
The author concludes that,
on average, it's best to tell all pickup owners to leave the tailgate up.
If you read the report, the most accurate answer to the question is: "it depends".
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