Suppose you filled thet 87 psi at about 70 degrF outside , so cold also inside tire temperature.
Then it changes 1 psi per 5 degrF.
So at 30 degrF 8psi lower is 79 psi.
My pigheaded camp states that you dont need to correct that , but are allowed when colder ambiënt temperature, but not when warmer ambiënt temperature ( 100 degrF ambiënt 87+ 6 psi= 93 psi, certainly dont lower to 87 then)
Main goal is to not overheat any part of tire-material when driving the speed constantly, for wich pressure is determined.
Then second , maxload of your tires is most likely given for reference-speed of 160 kmph/ 99 mph, wich you most likely stay way below. But check tires for that, speedcode Q and higher are all given maxload ( or loadindex) for that max 99 mph.
Suppose you drive max 75mph ( 120 kmph), you may add 4 loadindex steps( about 12% more maxload) to whats given on tire sidewall, to calculate with for the needed pressure, and leads to lower needed pressure .
So if you determined the 87 psi , based on real weighed loads and builded in a reserve for things like unequall weight, pressure loss in time etcetera, 81 psi wont give tire-damage, and you mind your speed a little, wich is not that difficult in the snow. Mayby only 50 mph possible, wich allowes adding even 8 li steps is about 25% more maxload, so even would calculate a lower pressure about 20% lower)
So in short, 81 psi, if determined needed pressure carefully 87 psi, no problem, certainly in the conditions now.
Rule of tumb for speed/ maxload relation is for every 10 kmph/ 6.2 mph other max speed ( wich you wont go over for even a minute) , 1 loadindex step different.
Higher speed> lower loadindex.
Lower speed > higher loadindex.
Already goes wrong for trucktires with lower speedcodes like K or L there 2 li steps per 6.2 mph.
In Europe, where I live, tiremakers give on trucktires, and Continental groop sometimes also on C-tyres ( eur equivalent of LT ,C stands for Comercial) an additional service description , is extra other loadindex for other max speed, so officially used system by tiremakers, in wich you see that rule of tumb.
And because laws of nature are the same in US and Europe, you can use this system also for LT and truck-tires.
