It’s winter again, we’ve had out first two snow storms, so it’s the time of year when I use plenty of washer fluid. To my dismay, prices have gone up quite a bit recently. Last year I was able to buy a gallon for about 95 cents, this year, the cheapest I can find is $1.4 and many other places over $2.. Not much you might say, but when there’s a lot of salt and dirt on the road, I can run through a whole gallon is less than two weeks. That adds up quickly. What to do?
In the past, I used to add water and rubbing alcohol, but that is not really cheaper if you follow the same procedure outlined below. Doing some online research for alternatives, I came across a great suggestion reading some discussion group. Simply dilute standard washer fluid with water. The reasoning is clear. Store bought washer fluid is typically freezing-safe down to -25 degree Fahrenheit. It rarely gets this cold here in Connecticut and probably most of the country. Canada and Maine is probably a different story.
The question is, how much should you dilute it to stay on the safe side? Based on past experience, zero degree F should be sufficient. You need to know the freezing point of methanol/water mixtures (methanol, or methyl alcohol is what keeps washer fluid from freezing) as a function of the methanol content, preferentially based on volume. The following table is taken from here:
| Vol % | Degree F | Degree F | Kelvin |
| 0 | 32 | 0 | 273 |
| 13 | 20 | -7 | 266 |
| 24 | 0 | -18 | 255 |
| 35 | -15 | -26 | 247 |
| 46 | -40 | -40 | 233 |
| 56 | -65 | -54 | 219 |
Based on this, store-bought washer fluid has a methanol content of approximately 39.4vol%. In other words, 1 liter of washer fluid contains approximately 0.6 liter water (H2O) and 0.4 liter methanol (MeOH).
If you want to increase the freezing point from -25 degree Fahrenheit to 0 degree Fahrenheit, then you need to dilute the mixture to 24vol% methanol. To compute how much water to add to achieve the desired dilution, it’s easiest to look at an example. In the above case:
0.4 l MeOH / (0.4 l MeOH + 0.6 l H2O) = 0.4
What we need is this:
0.4 l MeOH / (0.4 l MeOH + 0.6 l H2O + x l H2O) = 0.24
Solve for x:
x = 0.66 liter water
Therefore, to reach a freezing point of approximately zero degree Fahrenheit, add 2 parts of water to 3 parts of (-25 deg Fahrenheit safe) washer fluid (see disclaimer).
My car has a three liter reservoir, but the light (and an annoying beep) comes on at about 2/3 empty, so I need to prepare 2 liters of this mixture, i.e., I need to add 0.8 liters water to 1.2 liter washer fluid. When in a hurry, it is of course a lot easier to mix 50:50. That, in turn, will result in a methanol fraction of 20vol% and thus be freeze-save down to 7 degree Fahrenheit or -14 degree Celsius.