Oo nga... I've been looking this up, since I used to think 100% or 90% water is best... it's actually the opposite... 50/50 is optimum, as it raises the boiling point of water about 6 degrees... 70% coolant raises the boiling point 13 degrees.

There are special coolants for sale that allow you to use 100% coolant (waterless system). Ford uses this on the new Focus, and it has a lifetime guarantee. Some of the Miata club boys also run a waterless coolant system in their cars for track use.

Of note: these special coolants ARE NOT the same as the off-the-shelf stuff.

So, in answer to the original question? 50/50 ka nalang, dude.