Why does an F-body run hot at speed but stay cool at idle?

Cool at idle means the fan and shroud are doing their job. Hot at speed means the problem is somewhere else. Speedway's race-car rule of thumb: hot under yellow (slow) = airflow; hot under green (fast) = look beyond the cooling airflow.

At speed, check:

• Air path to the radiator: openings, debris, bent fins, and whether the shroud restricts ram air (performance shrouds use bypass flaps that open at speed).
• Water pump condition and flow.
• The tune: a lean air/fuel mixture makes cylinders run hotter, and ignition timing that's too far advanced or too retarded also adds heat. Speedway notes improper timing or air/fuel can make an engine run hot even with a properly sized cooling system.