Tailgate Air Resistance
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?
Ideas?????Comments?????
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?
Ideas?????Comments?????
0
This discussion has been closed.
Comments
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.
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?
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.
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.
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.
Driving in Florida on flat land in a 220 mile drive I got 17.8 miles to the gallon with the tailgate down. On my return trip same miles with the tailgate up I got 15.6 miles to the gallon. I have a 5.4 liter w/3.55 limited slip rear and overdrive transmission. I drive a F150 Lariat extended cab. The AC was always on and the windows were closed. My trip speed was between 80-90 mph. I do prefer driving with the tailgate up tho due to the structual integrity. Got lots of looks as I passed others in smaller trucks by. Also I used my cruise control as much as possible. My escort radar detector was also on
Based upon these observations there is a significant air pressure differential between the front and rear of the truck bed.
On the '92 F250, I was never been able to notice a difference in the mileage between tail gate up or down. The diesel have so much torque that the tail gate resistance is not noticeable to MY right foot. On other trucks that are lighter and with an engine with less torque there may be a benefit to the tail gate in the down position.
I do remember in either the '86 or '92 owner's manual, Ford stating that the tail gate is a structural member of the truck. (Or some such words.)
Finally, yes it is legal to run a truck with the tail gate down. The states where I've lived and can remember the specifics of the law, it is a requirement that anything that extends 36" beyond the rear of the vehicle must have a red flag attached. The tail gate on most trucks is not 36" high. Therefore the tail gate can not extend into the area where a red flag is required.
Rich
Of course, that's my testing, your mileage may vary ;-)