RichardL,
You say "now" I feel insulted as somehow I am changing my tune. I made no bones about your insult from my first post.
I never dismissed your original statement, that the T-cover makes no
significant difference in MPG. So if that makes you happier I agree with you on that. I don't see 1mpg which is about all one could hope for as significant, and neither do you, so on that we agree.
As for your absurd question about a 6" long bed, I don't know for sure, but I am pretty sure it would not have as much swirling wind over it as a bed that is 2' long, which would have even less than a 4' long bed, which would have less than a 6' long. But I don't know of any 6" or 2' beds out there, thus the question is rather meaningless. Is there a short enough bed that would have no wind across it...maybe... but again why assume, right?
As for your theory:
My theory is that the long 4 door cabin gets the air moving straight over the roof and the short bed may allow the air to go over the bed without forming the bubble? I think most of that bubble effect is lost with the shorter bed length and therefore there is really no significant difference with or without the tonneau cover, and probably no difference with the tailgate up or down as well.
I think you are wrong. As a matter of fact, I am sure you are wrong about the bubble effect. It forms. Whether or not it is significant is already agreed to above.
The bubble
does form on the short bed of the ST. It forms when the tailgate is up and the cover is off. You have seen this bubble yourself and attributed it to something else. You assumed it was because the short bed somehow kept these leaves safe from the forces of the wind, as if there was no air rushing across the shortened bed. I disagree. The leaves actually stayed in place because of the bubble effect. So the bubble effect does form on the short bed.
But why assume? There is a simple way to disprove the notion that there is no air behind the cab and on the short bed. You can do this this with a test. In this test simply put the tailgate of the ST down, and see if leaves or other lightweight objects fly out at highway speeds. If they do, there is wind rushing across the bed.
Hint: They will fly out... I have driven my ST in this way and they have.
That means that the aerodynamics of the ST and its short bed is such that wind does rush over the bed when the tailgate is down. However no such air flows across the bed and into the bed when the tailgate is up, by your own observation. Therefore we can see that there is a bubble with the tailgate up. These are good scientific observations. Granted, a wind tunnel would show for sure, but do we really need to go to that extreme?
The proof is that stuff stays put in the ST bed with the tailgate up, but flies out with the tailgate down.
So, my only original point was: If there is a cover on the ST then that cover will have more drag than the bubble; and more drag than that same air moving across the uncovered bed of the ST with the tailgate down...and less drag = > MPG.
You've done little to show me that the bubble doesn't form. I never debated the significance. I given reasons to show that it does.
TJR