"The manufacturer's recommendation:
The standard recommendation is to use a 50/50 mix of antifreeze and water. This represents a compromise between cooling efficiency and the ability to prevent the mix from freezing during cold weather. After all, the initial purpose of antifreeze is to prevent freezing.
But a 50/50 mix does not give the best cooling. For improved cooling in hot weather, we should use less antifreeze and more water, perhaps going to a 25/75 or a 20/80 mix ratio.
Everyone knows antifreeze prevents corrosion, and corrosion is bad. True enough. But, even a 10/90 ratio of antifreeze to water will serve the puproses of inhibiting corrosion, at least according to one source of information."