Hello people...
 I'm new to this forum and look forward to gaining lots of new and useful info.

 I hope this is the proper forum for this topic...

  I have been working for the U.S. military for the past 24 years as an aerospace engineer.  In the building where I work, there is a heathly mix of technical people;  (engineers, computer scientists, mathematicians, etc.)  A few months ago, some busy-body left a note on the windshield of my 1991 Ranger telling me that I would get better gas mileage with my tailgate closed.

 When I bought the Ranger in 1991, I decided to put to rest the question of whether the tailgate should be opened or closed.  I conducted a month-long test wherein I kept careful notes on the fuel consumption and miles driven with the tailgate open.  I then conducted another month-long test with the tailgate closed.  This is actual, real-world data based on my driving habits and vehicle.  I found that there was a 9.9 % improvement with the tailgate open.  This translates into a free trip to work with every tankful.  I have been driving my 1991 Ranger (which now has 180,000 miles) almost on a daily basis for the last 16 years almost always with the tailgate open.  (I only close it when I need to carry something.)

 In my book, empirical data trumps theories, speculation and models every time.

 My problem is that over the last few months, somebody with nothing better to do has been closing my tailgate during the day while my truck is parked.  (I guess he/she thinks they can save the planet one tailgate at a time)  Personally, I'd like them to keep their stinking hands off my truck!

 Prior to this person at work playing with my tailgate, I would read a column (like the one by Mr. Whale) and grin...  Unfortunately, some people take these articles as gospel.  What's next, is someone going to sabotage my RV  because it only gets 10 mpg?

 In my opinion, (for an older truck) the open tailgate issue is not a myth - but "your mileage may vary".  Undoubtedly, newer trucks (like my 2006 Sierra) would have more refined aerodynamics.  But the reader should understand that aerodynamics change significantly with velocity.  A truck optimized for 55 mph may have substantial drag at 70 mph.  That is why a test around a closed track at a constant speed provides basically worthless data.

 BTW, my solution to deal with the person who keeps closing my tailgate is to remove the tailgate altogether.

thanks for letting me vent...

- Jimmy  

yup... its a myth, i saw it busted on myth busters on time.. also truck trend did a article on it too


http://www.trucktrend.com/features/consumer/163_0506_tailgate_myth/index.html
Driving in a stock pickup with the tailgate dropped won't save gas.
:

Someone recently told me about a guy who puts his pickup's tailgate down to save gas. People who follow that practice point to improved fuel-economy figures as proof it helps. Although this may sound like common sense to some, it's a myth: Driving in a stock pickup with the tailgate dropped won't save gas.

We can't blame you for thinking that a flat wall of vertical steel at the back of your truck might present some additional drag, which could hurt fuel economy. However, a raised tailgate doesn't hurt aerodynamics; it helps.

In the old days of blunt-face pickups, most of the wind noise came from the front, but modern trucks draw more noise from wind around the mirrors and exhaust and road noise from behind. Air that circles the cab gets sucked into the bed where it forms a pocket and is for the most part removed from air that flows over the truck. If you've ever had a paper cup or leaves in the bed, as the truck's speed increases, they migrate into the forward corners of the bed and don't get pulled out over the tailgate.

I'm not an aerodynamicist, and I don't have access to the sophisticated equipment and extensive resources to prove this point about airflow, but places like the National Research Council Canada, Ford Motor Company, and DaimlerChrysler do, and they've all come to the same conclusion: Driving with the tailgate down decreases highway fuel economy, and removing the tailgate usually makes it worse. In many instances, it also increases rear lift at speeds, an undesirable condition in a vehicle with only a third of its weight back there to start with.

Putting a tonneau or hard bed cover over your truck is the best thing you can do for highway fuel economy. A shell also can help. Since most of these are relatively light, the extra weight must be worth the improvement. This is one reason something like an Avalanche (which has a bed that's partially covered) often gets better highway mileage than an open pickup with the same drivetrain and tires--and weighs less.