When we rebuilt the motor in our "69"chevelle this winter I also replaced the sending unit. With the new sending unit, the factory gauge reads 3/4 towards the hot when at full operating temp. With the old sending unit, the gauge always read 1/2 when at full operating temp. I have checked the the water temp with a mercury thermometer and it reads 192 degress, which seems to be about optimal to me. Does any one know if there is a way to add a resister or maybe change the sending unit to get the factory gauge to work correctly?