Refribing a 1984 300SD. Replaced the radiator, thermostat and all rubber hoses, etc.. after a full coolant flush with the engine drain plug . Took four passes until clear water came out.

My question -at at idle up to 45-50MPH the temp guage sits steady at 85C. However on the highway at 65-70MPH the temp gauge is at 90C. (No AC or heat on in both cases)

Been two weeks and no change. I had though the faster speed would cool the engine faster

ANy thoughts?