Has anyone ever compared the accuracy of a factory dash temp guage to an aftermarket underdash unit. For example when the guage reads half what is the actual engine temp in degrees. Or 3/4's or all the way to the right. Just curious, thanks....

I run both factory and an Autogage temp gauge and when the autogage is at 180 degrees the factory gauge is not quite up to the half way mark.

One way to check is to watch it during warm up. The T stat is fairly accurate... the guage will climb then level out, for at least a while, as the T stat opens. With a 180 stat, it will start leveling out around 185 degrees for example.

On my stock guage, 180 is the lowest end of the operating range on the stock guage... not up to the middle. They can vary a lot from car to car.

Get a cheapo cooking thermometer from the store; the kind you'd stick in a Turkey or something.

Put it in the top of the radiator, and once the thermostat opens, its temp will be pretty close to the engine temp.

Even my Equus gauge reads ~10*F high at operating temp.

Factory gauges are not accurate, nor intended to be. They simply show a function, and give it an acceptable range, that's it. That works for about 99.7% of the users (and GM didn't give them that- just lights that come on AFTER you've had some kind of system failure! ).

If you NEED an accurate reading, and most do not, then spend the time and money to install the proper gauges with their proper hardware, get it all calibrated, and you'll be good to go!