Temp gauge accuracy

RedFish said

No sir. A intake will never reach the actual water temp so long as the water is flowing.
Turn the engine off and watch the temp rise at the intake and temp sender. At that point the metals become more like a heat sink in that they absorb heat from the water. If the water circulation didn't transfer heat there would be no need for the circulation.

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This is probably minutia, but there will always be a temperature change at the interface between materials, so even stagnant water won't necessarily read exactly what the heat source is. Flowing water provides a different Tcold (in this case, the radiator, which, with forced air is really convective cooling and not radiative) and a better heat transfer coefficient since you are going convective and not conductive.

Knowing the interfaces and heat transfer coefficients you can figure out what the source temperature is measuring the manifold, but that sounds similar to some problems I used to do in college and don't have to do anymore :)

A thermocouple would probably be the most accurate way to check the temperature gage, but rigging it into the system might take some figuring.