I bought two caps, will see how well the tape holds and if it's ok, I'll buy the other two. I didn't have time to tape them yet, getting ready for a rally this weekend. I'm keeping the caps on front wheels - would having them on rears be better / worse for FE?
Also, once again I improved my best FE (6.25 lhkm, 37.5 MPG) by a very small margin. I had an early fill-up because I'm switching to different tires for the rally and want to isolate the FE on those from regular ones. I have no idea whether the tire spats or the belly pan had any effect, there's too many variables.
And finally, I have been working on a Kammback 2.0 for a few months, but could only devote a few hours monthly. I'm still not sure how I will attach it once it's ready, but I hope I can do it with zip-ties.