Something occurred to me about you guys' fat bike tires and the cold. When you pump them up the nearly adiabatic compression of the pump means the air you are putting in is not just the intake temperature but warmer. And as you go, since it's only nearly adiabatic, the pump heats up, which makes the air even warmer yet. So once you reach 7 psi or whatever, it's going to drop as it cools down to the indoor temperature - even prior to the dark morning outdoor cold. I guess there's no reasonable way to measure this with tools on hand. This is not unique to the fat bike tires but I am guessing it's proportionally larger than it would be with a little roadie tire in temperate conditions.

The idea that it feels slow with too low pressure is at odds with the prevailing wisdom, which goes more like, it it's not wrinkling, you aren't getting all the traction you could have. I have a bike with 3 inch tires and they feel too stiff in the mid teens; can't imagine running a 5 inch tire with 2x more volume at any pressure I could accurately measure on any pump gauge I've got.