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I hear what you’re saying. But I believe more may be at play here.
Throttle stop testing is one way. Another good indicator would be more tufts in the bed and on the back of the cab that would paint a better picture of the air currents. Also using pressure measuring sensors to measure downforce would be valuable as Julian suggests. I would assume it would add downforce. Which is probably a good thing in a pickup.
One thing is for sure.
A-B-C testing would be easy. (up, down, somewhere in between)
Just a flick of the switch.
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