I recently swapped my complete dial as mine has temp fuel & volts, which is what shug had been after. The fuel display did the same in his car. As my previous fuel gauge was ok, I'm going to disregard the wiring (which is all new) and the sender (which has been out for claibration).
I'm putting it down to fact my car is Neg. earth and this unit came from Pos. earth. It would be nice to fix, but I can live with this quirk.