FYI. I just got back from testing a full scale F-150 in the wind tunnel from 30 mph to 120 mph, and it turns out that gas mileage is worse when driving with the tailgate down.

 

Here's a site I found about this.
http://mars.acnet.wnec.edu/~ehaffner/did.htm

DF

 

I've heard that. I think I get better with the air on and windows up thatn window down and air off too.

-jeff b.

 

thats true... a couple of years ago (when i was an undergrad) i did a fluids project on that (we tested a 1/12th scale ram 2500 reg. cab longbed)... we found exactly the same thing....

with the tailgate down we saw an average of a 4-5% increase in CD for speeds from around 20-60 mph...

i should also note that with a bed cover (tonneau cover?) we saw decent improvements.... about 3-4% over conventional tailgate-up.

everytime i see somebody crusining around with their tailgate down (probably about 10% of the trucks in my area do this!) i just want to slap a copy of my report on their winshield... just shows how far myths can go. we talked to dodge when we were doing this research too, and they confirmed the same thing, also noting that leaving the tailgate down reduces the truck's structural rigidity.

heres some of the photos and figures from the report.... its not too professional (i was only a sophomore), but it should help.

http://www.geocities.com/jrmthof38/

 

wow... i just looked at the link posted by DF4.6, and it looks like they did the exact same test as us, only with a red truck (I'm at UT Austin Aero Dept).... our results seem to be a bit different, though... we saw a drag increase with the tailgate removed! this is just an example of how unpredicable fluid flows really are... probably why there are really very few well defined flows in engineering!

 

With the tailgate up a bubble of air gets trapped in the bed. That bubble is a has a lot less friction for the air going over it than if the air were slamming down in to the bed. I learned this quite a few years ago. I though everybody else knew it too?


-Jon