[Chevy Fleetside]

if you changed the sensor to a new one it might not really work with your gauge. I had that problem and used a 32 ohm resister to get it almost right. I used to heat up the engine and when the water circulated I used a thermometer in the radiator and it only go up to about 200º, this is when the needle was pegged and the resister fixed that problem. I had replaced the sensor with a so called one for a 65 chevy truck from one of the suppliers.

Also. Look in through your radiator cap and see if the cooling tubes are clogged. That will do it also especially on the freeway.