I use Schwalbe Marathon Winter 26x1.75 tires at around 55 PSI all the time. I have run them at lower pressure, but then they feel sluggish on the pavement and the last thing I want is a pinch-flat when I'm out after a 5" snowfall. I have also found riding through 2-3" of powder is manageable with these tires, but much more than that, and it's not really fun anymore. Plowed or snow-packed streets are fine.