I thought those pencil type tire gauges weren’t much good. I tried one once, and found it varied in its reading moment-to-moment by a few PSI. But I admit it was cheap. Are some of them better? I ended up using a dial gauge (it’s consistent, though I don’t know its accuracy), which matches the readings from the dial gauge built into my Schwinn bicycle pump.
The guy at Goodyear had an electronic gauge that read to a precision (I don’t know the accuracy) of .01 PSI (I think). But I’m guessing they cost a lot more. Plus it required electricity. 0
I suppose if you wanted to be extremely picky, you would adjust the ideal gas law calculations for external barometric pressure (because it is the difference between internal and external pressure that matters most), and take into account the tension in the tire belts (which would be hard to figure out).
And I suppose that if you did want to estimate internal temperature, the use of externally measured temperature might not work as well for a tire with deep treads, like mine.
I was once told that on snow, you should drop the tire pressure by a couple PSI. Presumably to increase the area of contact, so the treads can grab and push against more snow? But I guess that is a bad idea on ice, where I guess you instead want to increase penetration depth, which means you want more pressure per contact area. (Which means that my desire for high road clearance and good snow performance, but which implies taller, wider tires, which increases contact area, is possibly counterproductive on ice.) I’ve never found a good solution for driving on ice, other than driving slowly and carefully. 4WD or AWD do help some, and having a little extra weight in the vehicle helps a little too.
To make it worse, in the areas I sometimes drive, we often have scattered pavement/snow/ice. To increase this thread drift, what do you folks do as far as tires for such conditions? (Of course, most of the time, I drive on ordinary dry or wet pavement - so it’s not worth extreme solutions, like tank treads. 
Now that I think of it, I wonder if tank treads - which have studs - are street legal most times and places.) Full time chains are very inconvenient, and impractical in patchy pavement/snow/ice, and sometimes are impossible to put on when you are stuck, though at times I’ve used emergency chains to get unstuck. (If you want to XC ski, you usually have to drive during snow emergencies.)
E.g., if I had a real 4WD truck again with full differential lock selectable, would that be significantly better on ice than AWD? It’s certainly not perfect - I once spun out such a truck, in 4WD mode, on an ice covered bridge that caught me by surprise, while driving only about 15 mph. (And I once needed my traction mats on a dirt parking lot that turned to mud in the rain - and that was on those Firestone (275 R15? It’s been a while.) Wilderness AT tires. Though - it was on a pickup truck, which didn’t have fully loaded weight, especially in the rear cargo area. Also, I once pulled over onto a place that had ice covered grass, which I think may be worse than ice covered pavement. I needed a tow truck’s help for that.
