Last post on Jan 15, 1999 at 1:24 AM
You are in the Pickups - Archived Discussions
#1 of 21 fish6
Dec 01, 1998 (9:47 pm)
Can anyone really tell me if the tailgate is truly
responsible for lowering fuel mileage? Some say
yes.Some say no, because of some air bubble that
circulates around not letting air to hit the
tailgate?????? Has anyone seen or use a device I
saw on some web site once that angles down from the
top of the tailgate at about a 45 degree angle
towards the box which allows air to flow over the
tailgate, rather than hit it squarely?
Dec 01, 1998 (10:19 pm)
Actually, you will generally get better mileage with the tailgate up. This causes a mass of 'dead air' to accumulate in the bed, which acts somewhat as a buffer in the same manner that a hard tonneau cover does. Lowering the tailgate really messes up your aerodynamics.
Most folk who have tested with the tailgate up and down have, quite frankly, not done very scientific testing. The folks at Science News and a couple of other periodicals have all found that you will get better mileage with the tailgate up, all other things being equal.
Note: this does not take into effect the weight savings from eliminating the tailgate altogether, although I doubt it would make much of a difference.
Dec 01, 1998 (10:20 pm)
Oh -- and the air does not really hit the tailgate while you're moving due to the dead air patch (in the same way that dust accumulates on ceiling fan blades). I have my suspicions that the 45deg block is merely an expensive gimmick of little or no value, besides getting in the way when you wish to use your truck.
#4 of 21 fish6
Dec 01, 1998 (11:06 pm)
stanford...Thanks for the info.....makes sense!
#5 of 21 cobra4
Dec 02, 1998 (7:21 am)
Back in the 1980's one of the magazines ran a test
(Pop science? Pop Mech?) and claimed on an S-10 pickup that you got 1 mpg better with the tailgate
DOWN than with it UP. That is the opposite of
what stanford's articles said-anyone have anymore input?
#6 of 21 stevek
Dec 02, 1998 (12:30 pm)
On my '97 Chevy K1500 shortbed I have a soft cover. At highway speeds the front (toward the cab) cover bubles up and the part just before the talgate is pressed down (so much that I put a piece of wood under it). So I think the air would hit the talgate, therefore increasing drag and reducing milage. So much for scientific observations.
Dec 03, 1998 (1:35 am)
cobra and steve,
Don't forget, a truck is designed and wind-tested with its windows shut and the tailgate up (closed).
Steve, you are correct that some air hits the tailgate, but that is what creates the air bubble in the bed. Picture two airflow lines going over your truck, one about six inches higher than the other. The lower air line goes over the roof, into the bed, hits the tailgate and doubles back towards the cab. The higher air line must now flow over this retreating air in the bed, and clears the tailgate to get past it. In simpler terms, the air that hits the tailgate forms the bubble, allowing the rest of the air to go past the truck. If you left the tailgate down, two things occur that will disturb the airflow: the additional drop in height (an additonal 18-20 inches), combined with the funnelling effect of the bed sides. Air does not like to be channelled. Remember the top of your tailgate is pretty much in line with the base of the windshield, and air is coming around the cab as well as over it.
#8 of 21 cdean
Dec 04, 1998 (5:10 am)
without getting complicated, a tailgate up causes a large flat surface which a low pressure pocket of air forms (directly behind the tailgate). this is pressure drag. with the tailgate down, that area is not there, and the pressure drag is gone.
you still get pressure drag due to the cab, but not as much, because there is smaller area of your back window, AND, the presence of the sides of the bed cause turbulent flow. turbulent flow actually causes the "streamlines" Kcram mentioned to flow closer together, and reduce the low pressure wake areas that cause drag. (so much for not getting complicated)
i haven't run a scientific study, but i ran a couple of tanks with the tailgate down, and got slightly better mileage.
Dec 04, 1998 (2:30 pm)
I vaguely remember watching a program on the new NASCAR truck series dealing with whether or not they should allow or want to race the trucks with out tailgates....
Recollection was that there were slight increases in top speed and reduced lap times without the tailgate at some tracks, but at others running with the tailgate up was an improvement.
I am certain that I read another article that stated that the worst mileage comparing tailgate, no tailgate, and a 'prerunner' net was that the net was the worst case. I always run my trucks with the tailgate on and closed. I do tend to get better economy with the bed fully loaded with light stuff.
Dec 13, 1998 (2:35 pm)
Ford F150 owners manual says that driving with the tailgate down would adversely affect your mileage....I tried it all different ways, open closed, w/airgate, and it didn't make any difference that I could tell. IMHO don't worry about it. You will save only a couple of bucks a year in any event.