If you decide to mix up your own calcium chloride solution, you need to know that, while the freezing point of CaCl2 + water solutions starts decreasing with increasing concentration, it reaches a minimum of about -45F at a concentration of only 30% CaCl2. Above that concentration, the freezing point actually increases -- and not by a trivial amount. At a concentration of 57%, the freezing point is ABOVE 32F -- something like 42F. I was very surprised when I saw that some water solutions could freeze above 32F but apparently it can happen!
Google "calcium chloride-water phase diagram". I have a document with the phase diagram, but, since I don't know if it's copyrighted or not, I won't attach it. The title of the document is "CaCl2 solution properties.pdf" and is available from the Occidental Chemical Corporation.
The solution used for tractor tires probably has other stuff in it to depress the freezing point, along with rust inhibitors. This application is one where DIY probably is not a great idea.