2 things come to mind. I had you r same problem on my last set up. I was using a 4' firestik 2. It was bad from the factory I stripped it and kept checking cont all the way down. It finally read right about 1 inch from the base. But you have borrowed another set up and found the same thing, so that should not be the issue. I would look at your chassis grounds.
Your mount and surrounding area may all be good to go on the same ground plane but it sounds like you have an issue further down the line between your mount and your cb. Bad or corroded ground strap?? missing ground strap/wires??
If you have a roll of extra wire I would rig a ground from the screw on the mount to the neg post on your battery and try your swr reading again.
If you get bad swr with the rigged ground then I would look at the cb itself. It doesn't happen often but keying up with bad swr can jack up a cb. You may have damaged your when doing stuff earlier, or it could be bad out of the box
right now I'm running this due to the fact that we go exploring in the jeep then go hike new spots , so I can have a hand held and a vehicle mounted cb in one unit. works very well once tuned. If you end up looking for another cb see if this might fit your bill.
it's their version of the cobra 75, I believe the model is 822-75
