auto meter gauge

Are your wires (hard wired) into the guage at the back or did you wire it up? To me it sound like your wires are crossed and your guage is reading your voltage supply...

On the back of your guage ensure that wires are connected correctly, (I) is to 12V ignition on, (S) is to Temp unit and G ground.

If it's correctly wired....take it back.