Frank...
hmnn... more thoughts... (ground is the bottom of vehicle excluding tires)
lets think about this a little more thouroughly...
if the bed is REALLY short we can say it basically doesn't exist. assume equal air over top and bottom (invalid assumption) and what you get is a pull backward. if you have a LONG bed that is closed off on top you have a lower pressure zone above the bed which would cause air flow to be uneven and cause a pull both back and upward.
By removing the tonneau cover we now increase the "cab to bed" drop off, the bed to ground drop off remains the same (since the tailgate is still up); however the tailgate now acts like a scoop and the airflow sepparates. With the tonneau cover ON the airflow sheds from the cab and drops down towards the bed which increases the airs volume and decreases its speed (bernoilli), further more, the air comming off of the tail is forced up by the higher pressure air from under the truck which causes the air to collide with the rest of the airflow, this increases the drag and lift coeficients. with the cover OFF the airflow is split at the tail and has the opposite effect, the air inside the bed is flowing rear to front and heads upward at the cab. These airflows join without much tubulence or decline in pressure. Most drag and lift occur at the tailgate when the airflow separates.
Witht he toneau cover off, and the tailgate OPEN. The cab to bed drop is maximized and the bed to ground distance is minimized (essentially zero). The effect is basically the same as with the tonneau cover on except the relative lift is higher, and the relative drag is slightly less. Most lift occurs at the tail, and nearly all drag occurs at the cab/bed junction. (as a side note, youd be suprised how much additional lift the small air slot between the bed and tailgate can generate when the tail is down)
In conclusion, unless you are driving REALLY fast (ProTruck circle track) here arre the basics...
Most lift (low and high speed): tail down.
Least lift (low and high speed): tonneau
Most Drag (low speed): tail up
Most Drag (mid speed): tonneau
Most Drag (high speed): tail down
Mid speed being 55-80 mph or thereabouts.. i doubt it matters, maybe if you factor in lift as in decreasing the trucks weight??? But as it was posted earlier, better to remove the tailgate altogether.
for racing applications the lift issue becomes more important than the drag. i am sure we could kill the subject a bit more if we cared to.
Jay
