I am going to make my own but think I read somewhere that either methanol or ethanol can be bad for the car. All the internet recipes call for three cups ethanol, 10 cups water, a dash of liquid detergent. The storebought kind contains methanol. Are both safe to use? How cold will the above recipe be able to go before it freezes?
Since the stuff cost 99 cents a gallon, and you mix it one to one with water, why are you trying to mix your own?
Why would you want to waste three cups of ethanol on road dirt? Get some Hawaiian Punch, club soda, and do the mixage thang.
I am wondering that too. Of all of the things that you can improve and make cheaper at home, windshield washer fluid would rank with refining your own gasoline in my estimation.
Apparently, it sounds like people do it from the OP. Can you give us what you are trying to accomplish?
I work nights. It has been snowing here slightly. About 5 miles from work tonight, I run out of washer fluid. I stop at 5 (yes five) different places in the next 5 miles. All of them are out of Washer fluid. When I leave work in about an hour, the road conditions will be the same, and I have about 60 miles to go. The outside temperature is about 10 F., so I don’t want to just dump water in. I would prefer to be able to see during this time. After looking it up on the internet, I go to the little convenience store nearby and buy some rubbing alcohol. We have both water and liquid detergent at work. And that, my friends, is why I am spending twice as much to home make the stuff than it costs me to buy it at the store.
Only if you drink it while driving 
I get roughtly 10F for that mixture (minus the detergent). The detergent will drive it down lower, but in order to know how much we’d have to know the composition of the detergent and the size of a “dash.”
Bottom line - put another cup or two of ethanol in there, and you should be fine. Stay warm.
That’s the mixture, not you 
You also asked about methanol. It is certainly not safe to drink. Industrial methyl alcohol is pretty toxic stuff. You don’t want to breath the vapor, or get any on your skin either.
In the long term, I think it will also damage rubber seals and hoses.
Gah. I just realized you said that all you had was rubbing alcohol, which is isopropyl alcohol. Higher molecular weight means less freezing point depression. Make that a 15F freezing point for your given mixture. If you use 5 cups rubbing alcohol, that should get you down to 5F.
Disclaimer: I am not a certified chemist, I am not a chemistry major, I could be wildly wrong or have made computational errors. Take with grain of salt and do not operate heavy machinery.
Don’t forget you’ve got a significant wind-chill factor to worry about.
Try the “Rain-X” windahield washer fluid. Comes ready mixed or as a concentrate to add to ordinary non-freeze washer fluid.
Ordinary non-freeze washer fluid is often on sale special for 59 cents a gal.
Rain-X not only acts as a defrosting agent but makes for good visibility by preventing water from sheeting on the windshield.
Try it, you’ll like it. Don’t leave home without it!
Or, if you’re hung up on ethanol, buy a nice scotch and pour it in there.
Wind-chill does not matter when the question is freezing points of liquids and such. (for such read human flesh)
If it’s 5 below, but -15 with windchill the windshield fluid mixture will get colder faster, but it won’t get below -5.
Only if his car is covered in human skin.
I second this reccomendation. I picked up a gallon at Wallyworld for $2, and it works great. Cuts right though the film of ice still remaining after I scraped off the ice.
Ah, but if it’s a cold, dry climate evaporative cooling can cause the washer fluid to freeze at ambient temperatures higher than the nominal freezing point of the liquid. Wind, or the apparent wind caused by the motion of the automobile will add to this effect.
Rubbing alcohol is already diluted.
The stuff we have here is 50%.
Alcohol will also do a number on your paint.
good luck and be safe.
Hrrmm? I buy the blue kind during the summer and the orange -30 kind for the winter, and pour it in straight. I was supposed to dilute it??
Solution, drive in the direction of the wind and at the same speed to cancel it out.
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- FYI: you only need the commercial stuff with alcohol in it if the temperature is going to get near or below freezing. If it’s warm where you are/when you are there, what works better is to put one drop of regular dishwashing detergent into the empty fluid tank, and then fill the tank with warm water (use warm so the soap will mix well with it). You will see in use that it cleans much better than the alcohol stuff.
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- FYI: you only need the commercial stuff with alcohol in it if the temperature is going to get near or below freezing. If it’s warm where you are/when you are there, what works better is to put one drop of regular dishwashing detergent into the empty fluid tank, and then fill the tank with warm water (use warm so the soap will mix well with it). You will see in use that it cleans much better than the alcohol stuff.
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According to the book “Better Basics for the Home” by Anne Berthold-Bond, a 3:1:1 combination of water, rubbing alcohol and glycerin is good up to -28 degrees. But I think the DougC’s soap suggestion is probably the best, greenest and cheapest.