If you are performing a voltage drop test and you find that the ground side shows a high resistance or high voltage drop, one of the first places to check is the ground location on the vehicle. Look at your wiring schematic manual to determine where the ground is for your particular vehicle.
Once you have located the ground, remove the ground eyelet and clean the surface on the frame. Sometimes, the eyelet may be damaged enough that you need to replace it. After thoroughly cleaning the surface and the bolt head, then clean the second ground, which is the static ground that goes to the filler neck. Once all of the rust and scale is removed, reinstall the ground connection and perform your voltage drop test again on the ground side.