The picture below shows the flow using a net to replace the tailgate. The characteristics are the same as the "Tailgate removed", however, flow seems to disturbed or mixed as it exits the rear of the bed. The net causes more drag than with no tailgate. The drag coefficient with the net in place is 0.46. This is a 6.71 % increase in drag from the baseline condition.
Please add your links or comments about this old husbands tale.
BTW, WITH the tailgate installed and up the water tunnel looked like this:
In other words, removing the tailgate hurt aerodynamic drag.
The picture below shows the flow for the pickup with the tailgate removed. The characteristics are the same as with the lowered tailgate, however, the flow has no straightening mechanism at the rear of the bed. This is a cause of the higher drag coefficient. The drag coefficient without a tailgate in place is 0.447 which is a 3.47% increase in drag from the baseline condition.
edit: I just read what you wrote again. According to the site I was looking at, you are right.
The picture below shows the flow over the truck with the tailgate lowered. The characteristics are the same as the baseline, however, flow is allowed to pass through the rear of the bed. The lowered tailgate helps to straighten the flow behind the bed. The drag coefficient with a lowered tailgate is 0.414. This is a 4.17% decrease in drag from the baseline condition.
have u guys seen mythbusters? there ex-special FX pepole and test out myths to see if there true or not. on the discorvry channel.
taken from a mythbusters site:
Tailgate: Up or down?
Myth: Driving your pickup with the tailgate down gives you better fuel efficiency than with the tailgate up.
Adam and Jamie were each given identical, new model pickup trucks. They both had equal mileage, same tire pressure, and 30 gallons of gas. Jamie drove with the tailgate up and Adam the tailgate down.
They have to maintain the speed limit
All acceleration must be done by cruise control
Windows up, A/C must be exactly the same in both cars
After 300 miles there didn't appear to be much difference in the gas consumed, but after 500 miles Adam (tailgate down) ran out of gas. Jamie made it another 30 miles before he ran out of gas. This result was the exact opposite of the myth.
Water tunnel visualization
According to the experts, a circular pillow of air forms behidn the cab of the truck when the tailgate is up. This "separated bubble"/"locked vortex flow" keep the faster moving air from contacting the truck, and thus reduces drag. With the tailgate down, the bubble breaks down and is no longer able to keep the fast moving air out, increasing drag.
In their scale model with the water tunnel, they were able to see that the increased drag. With the tailgate down, the particles in the water were dropping down and hitting the tailgate.