Fuel economy turned out to be around 27, but that included some adverse conditions, so it doesn't seem to be any different than with the kamm on.
Do you think adding a piece of coroplast to the trunklid to lay down the spoiler angle would be more effective?
Edit: Taking some advice from
here, it seems like adding coroplast to lay the spoiler down to a 30* angle and meeting the roofline at a 12*-13* angle seems ideal. I think I'm pretty close to that, after some quick Gorilla tape/coroplast work. We'll see what happens.
Update: Okay, dropping the spoiler angle to 30* and extending it to meet the roofline at 12* seemed to make things worse. The needle dropped much faster than it usually does. There wasn't any wind, so that wasn't a factor. I'll pull it off and try the rest of the tank without a spoiler.