how do i tune a truck compass?

I have gotten a truck/boat compass and am trying to get it to point accurately in my truck. It is to be mounred on the center hump (it will be in the way on the dash). It has North/South and East/West adjustments which I have been playing with, and they do make a difference though they haven't gotten it accurate yet. Can it be tuned totally accurate? If not, what is the closest I can get it?

Go to your local airport. Point your truck along the runway. Then adjust as necessary. Or find a friend with a GPS.

I have a gps to tune it by and know it does not like West. Spinning the screws on the bottom of this thing hasn't cleared it up. Where do I go from here?

Remove it from the vehicle and see if it works outside. If so, then you are getting some magnetic or electrical interference. If it doesn't work outside, then its bad.

It works fine outside the truck so I know it's good. It just needs the correct binnacle adjustments (those screws on the bottom) to make it see straight. My question is how to adjust it so it will point West, and how accurate to true heading can I get it