Oz Pony offers some good advice .
If your temp is really ok you can insert a resistor between the sender and the gauge they only cost a few cents each(**** Smith or Jaycar) so buy a few diferent ones 2,5,10,20,50 and see which one or combination will get your gauge to read where you want it.I use a 10 ohm on my 66 hope this helps.
Wimblton white 66 coupe C code 289 auto
97 Jeep Wrangler 4.0 auto, lift ,winch,33's etc
04 Holden VY SS 5.7 auto (wifes)
88 KLR 250
Please feel free to ignor all advice/information offered
I could be wrong...... I have been before.