OK, so your grounding to aluminium which is frankly crap.
To get a decent CP, mount to as much steel as you can find - on a 'fender it's rather hard, but you do sit on a sodding great lump of the stuff - the chassis - Now mounting an aerial on the chassis is not the easiest thing in in the world, so you have to do the best you can....
Remember, copper is the best CP and Steel is also quite good.... get my drift here?
You also need to remember that SWR changes with frequency. So, you set your CB up on, say Ch2 and SWR it from there - by the time you get to CH40, the SWR will have changed - its a compromise, so make the most of it - SWR your rig in on the middle channel that you have: eg: 40ch set, SWR on 20. That way, your going to be in a better position working +/- either side of Ch20 from an SWR point.
Next lesson will be about cable 'twix rig and antenna