Running hot, I think.....

1965 Dodge Dart 225 slant 6

Re-built engine
new water pump
new thermostat
Radiator cleaned

I have been noticing that around town the temp gauge shows a little hot. As soon as I get on the interstate the gauge goes all the way over to the right pegging in between the last mark and second to last. Today I had the radiator boiled out and it went from 10.9 gallons per minute to 18.8 gallons per minute. That radiator should be around 20 gallons per minute so it is operating fine. I install the repaired radiator and same thing happens. Does anyone think it is the sending unit? Thermostat? with the re-built engine I also put on a new waterpump. Any ideas?


Thanks as always.