[brucepick]

Part of this discussion is based on the idea that less cooling air is needed at high speed.
Which is why the perforated panel holds some promise.

False premise.
Actually at highway speeds cars (my car, at least) need MORE cooling air.

I do appreciate the explanations of how/why the perforated panel tends to pass a lower percentage of air at higher speeds. Good stuff to know.

I built the grill cover panel you see in my avatar this past Sept. and installed it just before an early October road trip, 300 miles each way. With temps around 75 deg. F, it was fine in town but got a bit hot on the highway doing mostly 60-65. Frequently the temp gauge went up about 3/16" to 1/4" over the center point where it usually rests, and the elec rad fan came on to help cool things down. A few times, I ran the heater to cool it.

With cooler weather its been fine all winter. I'll be building a new one with a larger opening for summer.