If your vehicle is showing temperature gauge drops to cold when driving at highway speeds, here are the most likely causes and the parts you'll need to fix each one.
The thermostat regulates engine temperature by restricting coolant flow to the radiator until the engine warms up. If it fails and sticks in the open position, the massive amount of airflow at highway speeds will overcool the engine.
A previous owner or mechanic may have removed the thermostat entirely in a misguided attempt to 'fix' an overheating problem. Without it, coolant flows continuously, preventing the engine from maintaining operating temperature at speed.
The sensor that sends the temperature reading to the dashboard gauge may be failing. While this usually causes erratic bouncing, a failing sensor can drop to cold if its internal resistance drops out, or if the wiring connector is loose.
If a mechanical fan clutch is seized or an electric radiator fan relay is stuck closed, the cooling fans will run constantly at full speed. Combined with highway airflow, this can over-cool the engine.
If the coolant level is extremely low, an air pocket can form around the temperature sensor. Because air transfers heat poorly compared to liquid, the gauge may suddenly drop to cold, even though the engine itself might actually be overheating.