Aftermarket water temp gauge reads low due to wire length

How did you determine that the gauge reading was 10 degrees off? When I replaced my temp prob it seemed to run hotter than the OEM prob. It bothered me for a while because the fan clutch seemed to come on much later. I decided to shoot the prob itself and the surrounding area with an IR gun. The readings were 15 degrees lower than what my dash gauge was reading. Also shot the thermostat housing it was also 15 degrees lower than the gauge reading.