They cost mileage. Here are some links:
http://www.awtrucks.com/tailgateup.htm
http://amos.indiana.edu/library/scripts/tailgate.html
http://www.findarticles.com/p/articl...05/ai_n9439015
http://www.fordf150.net/misc/tonneau...pg-article.php
http://blog.kevindonahue.com/archive...s_mileage_.php
http://www.pace-edwards.com/gasmileage.asp
http://www.popularmechanics.com/auto...tml?page=5&c=y
http://www.epinions.com/msg/show_~th.../forum_id_~124
http://trucktrend.com/features/consumer/163_0506_ww/
To sum it up, trucks are designed so that an air bubble forms in the bed, kind of like an invisible tonneau cover, and the air flows semi-smoothly over it. Drag increases with the gate down or removed, also if a net is used. For optimal mileage gains, get a tonneau cover. Bottom line, you probably won't notice the difference either way, unless you get the tonneau cover. However, if you check your mileage to the 4th digit like I do, you will notice the loss/gain.