At CB frequencies the vehicle does not provide a great ground plane, so I wouldn't be overly concerned. You do need a solid RF return path and that is not necessarily intuitive like DC. A #6 wire might seem like good but it's not really any better than a #14 unless it has a very high strand count. The best hasty antenna ground material is the braid from extra coax and a good braided and tinned copper braid is the best ground material for RF.
BTW, talking or not, on CB it's always important to keep your key-ups short while tuning. Even if you do not talk the radio is transmitting a carrier, which is 3/4 of the power in the signal. Even if you never say one word you'll burn up your transmitter if you leave it keyed for long periods. Try and make the measurements in very short bursts, just a second or two and leave a bit of time in between.
If your SWR measurement is higher at 40 than 1, you antenna might just still be electrically too long. Try turning the tuning screw down to its minimum length and see if the SWR changes a lot. This will tell you if you're looking at a tuning problem or a more fundamental problem of a short or bad coax or something.
One thing that's easy to check is that the PL-259 connector has good conductivity between the center pin and the antenna center, which is the inside threaded part on your mount. Check that it does /not/ have DC conductivity between the center pin and the vehicle body, indicating a short in the connector, coax or mount. Then check that the outside of the connector has good conductivity to the vehicle.
Also, you might have to scrape away a little paint from under the antenna mount to make sure you have a good path there. You want it touching bare metal.