I have 42 mm tyres, Marathon Winter. Which tyre pressure, perhaps as a starting point, would strike a good balance between traction/grip on ice and puncture resistance? 50 psi?

I'm not aware of any non-studded tire that provides traction on ice.

Marathon winter is studded.

Ah, thanks, wasn't aware.

I have them in 700x50 on my hybrid, I run 30 psi when it's not overtly icy out and 15 when it's icy.

I run 26"x1.6" Suomi/Nokian W106s studded tires which look similar as to number of studs and placement, but with a slightly more aggressive tread pattern and depth than the Marathon Winters. Given the differences in size and type, here's what I do:

Dry: 65psi (max rating)

Ice and/or packed snow: 30-33

Deep, loose snow 20-25psi This still works well for any ice, but it's a real drag on dry sections.

Rim width and rider weight? Puncture? They are really beefy, should not worry about that. Grip? Run less. Sit on the bike and have the tires squish a bit. Me? 40 mm nokians, skinny rims, 235 lbs, usually run 30-35 or so.

max pressure on dry roads. when there is some precip just soften them up a bit

Look on the tire sidewall for the recommended max and min pressure.

When it's pretty clear out and your only concern is small patch of black ice run them at the max pressure listed on the sidewall (I think it's like 65psi or something).

When it's grittier out run them at the lowest pressure listed on the sidewall (I think it might be 30psi? I'm not sure offhand).

Thank you all for your advice. Bye the bye, SMW tyres (42 mm) are 30 psi up to 70 psi.

Meanwhile, I’ve been running them at 40 psi when bad and up to 65 psi when danger is much less.

More important will to learn how to deal with types of snow, which risks I can take etc. Because some situations – even with studs – are quite challenging.

My new Marathon Winters are rated up to 85 psi. I've ridden them twice so far, in "snow that never really happened." If this is going to be like the last couple of years, there will actually be minimal snow or ice on the roads, and the studs are in case of a sudden change of weather, an ice storm, or patches of ice here and there. I think the higher pressure is where I'll stay until I have a reason to do otherwise.

My 35-mm Marathons are rate for 35-85 psi. I probably run them at 60 psi.

i just my marathon winters on yesterday - i'm running them at 85 psi. they're mounted on 19mm internal width rims. my biggest concern is my weight. at 265, if i run them much lower, i'd be afraid of pinch flats. not that i'm hopping curbs or riding aggressively but still... anyone with a similar weight as me care to comment on how low you run the pressure before you get pinch flats?

mounted up my 40s. massaged them onto the wheels as I pumped them up slowly, then brought them right up to their max 70psi. test ride around my immediate neighborhood on plowed streets w lots of 2" thick ice covering large portions of the roadway including one steep hill climb. everything checks out perfectly

You could probably knock 10psi off the front for starters. If I'm playing around with tire pressures, I'll take glances at the bottoms of the wheels while riding to see how much the tire bulges at the contact patch. A little is okay, but if it's squatting a lot you may want to use more pressure or move up to the next tire size.

hitting the ice & snow covered paved trail yesterday, rode for some time at max pressure then decided I wanted more contact, so let out some air from the front & a little less than that from the rear. kinda like 2 seconds from the front & 1 second from the rear. felt like the right amount after getting going again. sorry didn't measure them