If you have gas fumes in the cab,you have a problem.Somehow you have a bad connection,or something is not hooked up I correctly.For years the in cab gas tanks worked out just fine.Now I see that many of you posting here want to relocate your gas tank to an under the bed unit,which usually turns out to be a costly operation.And it is usually money poorly spent.I have two old Chevy trucks,a 55 and a 65 and both are heavily modified,and yeah both of them still have the in cab gas tanks.No problems here,as they are hooked up correctly and work just fine.If you have fumes start checking things out,the tank itself,filler connections,and gas gauge hookup.
