I installed an aftermarket SunPro mechanical water temp gauge.I have it tapped into the intake manifold at front opposite the heater hose fitting.I have one of those Mr.Gasket hiflo stats 180 degree.The gauge is only showing about 155 fully warmed up.If I take a heat gun and warm the sender the gauge moves right up as long as heat is applied.The  factory dash gauge which has the sender screwed into head is showing about halfway between "öff" and midpoint of gauge which I think is pretty accurate.Is the water temp 25 degrees cooler at the point I have the sender screwed in or is it more likely the gauge is not correct?Should the water temp not match the opening temp of the stat which is 180?