Temp gauge reading may not be accurate

95 FLD 120 with a Cat 6nz. Had an issue quite a while back with the over temp buzzer going off on the dash. Figured out it was the sending unit for the buzzer. Installed a manual temp gauge, as I wasn't sure initially if it was the sending unit or it was actually hot, although I didn't think it WAS hot. Manual gauge is in the back of the head, electric gauge is on the thermostat housing. I get a 15-20 degree difference on the gauges, where electric higher. Electric gauge will read 200 and manual 175. Have changed dash gauge and both sending units on engine. No difference. Not sure if there is actually this big of a difference front to back of the engine. I would put the manual in the front, but not a quick job. Anything else that may be an issue?