Windshield washer fluid is indeed made of a mixture of water and alcohols to lower the freezing point. Available in most public libraries, the CRC Handbook of Chemistry and Physics, 56th edn, table D-237 gives freezing points of methanol-water mixtures, so a 1-methanol to 2-water, i.e. about 33% methanol has freezing point of -30C; 15% is about -10C. Table D-220 gives a variety of solute/solvent tables. But this won't likely solve your problem. I doubt you can beat the $2.50 per gallon price. [Editor: a quick google search gives a recipe of 3 cups Isopropyl alcohol, 2 teaspoons liquid detergent in 10 cups of water. You are likely to find that 3 cups of isopropyl alcohol costs more than $2.50, and it might not lower the freezing point of water as much as the commercial product. Best advice is to make the stuff last longer by diluting your gallon of $2.50 windshield washer liquid with water during periods of moderate freezing weather and certainly in summer. But when it's -40, you will need it full strength.]
It depends on the climate. If you live in a region where the temperature is below the freezing point you must have methanol in the washer fluid. In general you need 30 - 50% alcohol for a quantity of water. To this you can add a small amount of detergent. It would be better to test your mixture by leaving it in a container outside one night before putting it into your car. One possible warning: I have read that one of the alcohols (methanol or isopropanol?) can turn rubber white.
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