Funny you should ask...
I spent all weekend dealing with just this topic. I have a 3' firestik II that I mounted on the fender using the first bolt near the cab. I could not get my swr below 3:1 in this location. After replacing about everything, running a new ground and then having a cb shop look at it (they couldn't even figure it out) I found the easy way to fix it was move the antenna out to the second bolt from the cab and this provided a much lower swr. To get a 1. 5:1 swr I had to go with a 4' antenna mounted on the second bolt from the cab. I hope this keeps you from spending as much time and money as I used to figure it out.