meter match or equivalent

I got one of the cheaper ones, off e-bay. I just got it installed, and have some issues. I can't say if it is the unit or the vehicle. This is a pickup with a 30 gal tank. What I did was measure what the tank held from empty. Held 32 gal from where no fuel would come out the line at the input of the fuel pump (line disconnected from the pump, and connected to a electric pump, to jugs), to fuel in the filler tube. Ign on and set the "full" setting on the adapter. Pumped out 8 gal and set the 3/4 setting. Pumped out 8 gal, and set the 1/2 setting. Pumped out another 8 gal, and set the 1/4 setting. Pumped out the last 8 gal, and set empty. Connected the fuel line back to the stock pump, and refilled the tank. With the engine off/key on gauge reads full. Key on/engine run, gauge reads less then full. I haven't driven the truck enough, yet, but it seems like the gauge reads about correct at key on/engine off, but reads erratically lower, with key on/engine run. It might be an issue with varrying vechicle voltage. I need to run the tank down, and also check voltages. Will post back when I get a chance to do some trouble shooting.