You exchanged a larger mechanical clutch fan for a smaller diameter electric fan. Of course it does better at low engine speeds because heat output is low enough for the fan to handle it. On the highway the engine is putting out too much heat for the fan to handle it. My '69 has a/c and a 4 core brass radiator. Even when its 90 out the clutch fan keeps the temp below 220 in traffic. I have the factory 7 blade fan and a HD Hayden clutch.
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