[Peter Serio]

All those imported gauges are not tested/calibrated using a sender that is because aftermarket senders all vary because none of them are made to the GM standard. The imported gauges are not exact, they are copies and they are not all made accurately. They sell "new" gauges for many different (years) GM car restoration projects. Add to that some GM cars with gauges are labeled Cold-Hot (1968 GTO for example) and others are labeled 100-250 ('69-72) GTO. The trans am temperature gauge for 1970-72 was labeled: 100-210-250. When they added a restriction (catalytic converter) into the exhaust system in 1973/74 they changed the thermostat to a 195. Those Trans Ams have a temperature gauge that reads: 100-220-260.

There is no way the aftermarket (1/2 way around the globe can keep track of all these changes). Those people do not have a shop manual for your car over there. There are no cars like your car over there for them to go and look at!!!

The people who make new temperature senders are not the same people who make reproduction gauges (that never read true anyway). Modern aftermarket replacement senders are all "combined" today. For a car that is 40 or 50 years old, they can't spend a bunch of time on something they might sell maybe 100 or 150 of in a year. That is where they take what may have been a half dozen original GM #s and combine all those into one "replacement" part #. So right there we have another calibration issue.

I have been a GM mechanic for over 42 years. I can test & calibrate any temperature gauge (electrical) out of any old Pontiac with "factory" option gauges.

I make up a (real world) test on my bench which includes a 12 volt DC battery, 4 or 5 jumper wires, boiling water, a thermometer and the gauge plus several different senders in the water all at once to determine which one gives me the best readings. I have calibrated over 100 temperature gauges for customers (all across the USA) with lots of different year Pontiacs using this method.
As long as your GM gauge is NOT missing it's pointer; has not been dropped and broken, or left outside in the rain; there is a 98% chance that I can repair it here using USA made parts.

You can always PM me in case of any questions.