misreading temp gauge

What I would do first is get a ohms meter. Pull the sender wire off the sender. Check sender resistance to ground with one probe on the engine right beside the sender, then again with that probe at battery negative. General ambient temp, cold engine, 80 ohms or better with about the same resistance anywhere checked.
Next run the engine until that gauge gets to its wrong or 3/4 as you claim. Pull that wire from the sender again and repeat tests. Approx' 14 ohms resistance to anywhere ?