This is what I do (did) too

 

Pre-mixed on every car I touch.

 

Why? You are getting ripped off by paying the same money for only 2 quarts of coolant.

 

Water with impurities is not good for your car nor is distilled water that is why you have coolant of course and also to increase boiling point and decrease freezing point. Water has ions in it already ---> Fe2+(aq) + 2OH-(aq) ==> Fe(OH)2(s). Coolant needs to protect the engine from the flow of electrons to the liquid and between metals, right? I doubt tap water will be much "worse," if at all. Might actually be "better." I mean, what is coolant? Water with impurities that decrease oxidation and electrolysis and raise BP and lower FP. With all that being typed distilled water does conduct electricity worse than city and well water.

Interesting some one mentioned Fluoride. It's worse for you than for your car. About 1000 studies on Fluoride and how is reduces IQ and causes a type of bone cancer. 1 discredited study that states it's good for your teeth.

Using distilled water for coolant is not necessary as far as I know. They probably don't make the coolant using distill water anyway, if I had to guess.

The house pipes being clogged with substances in the water is a silly analogy because you don't have a 10,000 gallons of water flowing through your car every year.

Click and Clack don't recommend the use of distilled water for mixing coolant, if that makes any difference to some one.

 

I have been using distilled water since 1968. Never experienced a problem. My wife uses it in her steam iron, and it has never had clogged steam vents.