I've been working this same problem in a 49 GMC with a 54 Chevy 235 for the better part of a year. The electrical system has been converted to 12v and has the standard resistor/ballast in the dash/gauge circuit to keep the instruments operating at the original 6v. When the engine is started the temp gauge stays pegged to the cold side for about five minutes. When the 180 degree thermostat opens the gauge needle slowly climbs without hesitation to the max 220 degree position. I've done all the gauge/sender component tests and all indicate a good sender and good gauge component. Both 12v and 6v senders act exactly the same. With the guage indicating 220 degrees I've dropped a thermometer in the radiator and consistently gotten temps in the 180-185 range. Other than the temp gauge, I've never had any indication of overheated coolent. Lately I've been thinking about putting a rheostat between the sender and the gauge to vary the resistance and see if I can get the gauge temp indication to match the temp I'm reading on the therometer dipped in the radiator. Anyone tried this? If you've not tried it, what do you think?

Mike

Mike
'27 Chevy
'41 John Deere B
'49 GMC