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Hi Metro,
How about trying them at the front of the roof, about 6 inches back from the windshield, or even right at the top of the windshield? That is more in line with aircraft applications.
It looks like where you had them they were completely within the boundray layer and thus there was little impact.
Aerohead was saying that the flow is seperating there already. So, this makes sense. The idea at putting them forward, is that they may keep the cross section of the turbulent flow smaller.
On an 18 wheeler, my guess is that the flow is pretty laminar by the time it gets to the rear of the truck. That is, the turbulent effects have decayed, which then start up again behind the truck. I base this on driving next to trucks and watching my Prius mileage indicator. Cars are not long enough for these to have an effect on the roof above the C pillar, I do not think. Even though that is where the EVO uses them. The EVO has such a highly angled rear window, it may be pulling the bounadary layer on the roof down.
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