My dealer charged me $390 to install Rimguard in my rear tires before he delivered my new tractor to me.
The rears are size 420/70-24's (Industrials).
Filled to the top of the upper rim, you can see here in the frost line:
View attachment 629840
I don't really think you need to worry about that level of freeze protection in Seattle. Unless you're looking for the most weight density available. I think windshield washer fluid will work there without freezing, as you just don't get that cold along the coast. We use it here because our sustained temperatures can be at -40F for weeks at a time. The pic above was taken at -30F.