Water temp gage

[6863m]

I have started my engine a late 1970's 350 cu in and it runs well and I have put in my 62 gage package original with the truck. I also put an early sensor in the engine to match the electric of my gage. I did take the panel to an instument shop and had them checked. Now when I start the engine the gage goes from cold to full hot in less than 3 minutes. Yet my electric fan comes on and is moving a lot of air and using my lazer temp gage the engine does not get hotter than about 185 so the system is working but the gage acts funny.

Is there an answer for a gage that just goes direct to hot. It is not quick it does take 2 to 3 minutes.