On the bed rail of my truck, I got good SWRs with the Comet except on the 2m band. I use the 2m band a lot, so that was a no-go. However, on the roof of my car, I get SWRs < 1.5:1 on 2m, 70cm, GMRS, MURS, and -- to my surprise -- even 1.25m. I haven't had an opportunity to find out whether it actually performs on 1.25 meters, but at least it won't blow up my radio. So, yes, it's a ground plane antenna, but it was better than I expected without one.
As I said earlier, I thought the SG7900 was an NGP antenna when I bought it, and since it performs just fine where it is, I'm going to leave it. Excellent SWRs and farz on GMRS, 70cm, and 2m. I guess it is a no ground plane antenna -- at least on the bed rail of my truck.
I'm currently running a Comet SBB-224 on the car, and boy is it a good performer! It didn't like the bed rail either, but it really likes the roof of the car. It's officially a tri-band (2m, 1.25m, 70cm) but it really reaches out on GMRS as well. It has a 1.2:1 SWR on MURS, but I've never actually tried it, and likely never will. Still, it's there if I need it. The 2x4 actually has a little better farz on 70cm and GMRS, but not by a lot. That's why I'd like to know how it performs on 1.25m. Likely the 224 will go on the truck if I ever get around to having an NMO put on the roof.