The gauge is a "balancing-coil" type, with one coil getting full voltage, and grounding to the dash. The other coil grounds through the sending unit. Both coils have to function properly for the gauge to read right. Check the gauge-to-dash panel circuit for a high-resistance connection.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!