Have you removed the sender and jumpered the outer case to engine ground, reconnected the top lead, place the sensor in a pot of boiling water (just left the stove) and turn the key to run? What does the gauge show?
Its usually not a lousy ground or corrosion as those will give you colder readings.
This chart will help on the relationship.
http://www.nsifleet.com/pdf/Temperat...tanceChart.pdf
I use a VDO
https://www.vdo-gauges.com/sensors/t...-801-1-16.html
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1