Before installing the Auto Meter gauges or any gauges for that matter, I bring them in the house and put the sender end in a pot of water on the stove to test them, both Auto Meter gauges and an el-cheapo Auto Zone set were within 3 or 4 degrees when the water started boiling.....
I also do the same with a new thermostat and I let the water cool down and make sure it closes properly, I go thru the cycle 2 or 3 times.... I changed the thermostat in my race car a month or two ago and tested the new one and found that it would stick in the fully open position....messed with it a little and finally decided it was not worth trying to fix, threw it away and got another.....
BTW: makes my wife really mad to see me testing stuff on her stove!!!!!!!!!!!!!!!!!!!!!
David