The typical propylene glycol that is stacked up by the pallet load in late fall
is rated to -50 Fahrenheit. But they also make "100 below" -- it's just not as commonly displayed.
I use the -50 in places that are open but might still have water in them that I don't want to freeze (say, in the bilge of a boat where if it did just start to freeze it would simply expand and there would be no real damage). But where it counts, I use the -100. That's not because it will get below -50 where I am (or was...), but because the possibility exists in pipes and the like for the solution to become slightly diluted with existing water.