Good point. There's plenty of power and cooling capacity to spare on vehicles that aren't designed for fuel economy.
I've done 100% grille blocks and 100% radiator blocks (with the cardboard/coroplast directly against the radiator so air can't even come from around the sides) on my VW and my GMC and didn't manage to get the temperature to budge (with a couple extreme exceptions, and even then it wasn't anything to worry about).
Linear Mode
