50/50

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Personally, I wouldn't mix it 50 / 50 unless I lived in a very cold climate and parked my car outside.

Water is actually a better coolant as far as heat capacity (Cp), so I use only about a 30% mix, which provides a freezing point protection of about +5 F and a boiling point of about 254 F. Adding more antifreeze does not raise the boiling point very much, and really only serves to depress the freeze point.