There is a voltage regulator on the back of the pc board for the gauges. It limits gauge voltage to 7v, and serves the temp and fuel gauges. If it loses it's ground, it allows full voltage to the gauges, permanently warping the bimetals inside, causing a low reading on BOTH gauges. The ground is a screw on the pc board, and is shown in the FSM. The only cure is to swap out the gauges, and tighten the ground. With a normal gauge and stock thermostat, your temp gauge should be just short of half scale at normal op temps.

Not responsible for advice not taken...