Either the sending unit is incorrect or the gauge is defective.
The gauge in the '56 that sits in my garage was repaired by Bob's Speedometer. When the key is on, it goes to COLD and stays there until the coolant temperature goes to 180 and it moves a little off COLD. Then at 190 it will move 1/4 of the gauge and at 195 it will go to half. I've never seen it go any higher  than that. Have electric fan and only use it at red lights here in Hot 'n Humid Florida.

SIDE STORY: The temperature gauge that was repaired by Bob's Speedometer came out of the dash of my first '56 Ford Fairlane that I had when I was a  teenager.  I sold the car to a friend who wrecked it but form some unknown reason tore the dash out of the car and put it in his grandparents barn. It sat there in that barn for 58 years.
Last year, I was up around Detroit and found out the dash was still there and I gave the new owner of the barn a 6 pack of Bud Light for the dash and put it in the trunk of my 1956 Ford Crown Victoria and it now hangs in my garage here in Florida. It had the original temperature gauge, so it was sent to Bob's Speedometer to be repaired and it is now working just fine in the Crown Victoria. If that temperature gauge could talk....etc.
Thanks for your time,
Tim 
Hot 'n Humid, FL
33706

We checked the calibration of the sensor and it appears correct so the problem is the gauge.  At around150 degrees the sensor is reading the correct ohms but the gauge is pinned at hot.  Gotta call Bob's speedometer and ask him why.  

get a dash voltage reducer from 1957 and later forcer. i installed 1 in my 49 mere 12volt all gauges work now,even gas gauge

1956 is the only Year Gauges were designed to work on direct 12 Volts. The 1957 reducer can be used on 6-12 Volt Conversions as well as 1957 and later 12 Volt Systems.