Car overheats on the highway?

You give no info on your car. But my experience with my 1968 Olds Delta 88 (many years ago), late ignition timing cause a consistent overheating problem on the highway. With today's computer controlled systems, late ignition timing I believe is a thing of the past.

Also, check the serpentine belt. If it is routed incorrectly, the water pump could be made to run backwards. But I would expect the engine to overheat anytime with that problem.

I don't think it would be your radiator fan (if electrically controlled). There's enough air flowing over the radiator to keep it cool enough to where the fan isn't needed.

You dont sound like an fool, this could properly be a stable question. Your autos subject is that the coolant isn't circulating quickly sufficient at severe velocity, for this reason inflicting it to overheat. lots of the time that could be a foul thermostat, it being caught a million/2 way open. I see this truly generally. If that doesn't restoration it, then you definately've the two a partly plugged radiator or a failing water pump. lots of the time, that's the radiator if one in all those 2 are the criminal. yet replace the t-stat first, approximately ninety % probability of adjusting it.