Mazda unqualified statement is confusing and generally incorrect. Or just plain wrong on the 4psi over placard.
Ne'er on any car Ive owned over 50 years have snowtires provided good traction inflated over placard.
Never. In fact it was dangerous.
Since it doesnt usually dive below 10F often where I am in the early AM. I try to run snows at about 3 psig under placard
Cold (measured typically at 18deg F.) That ends up around 28 - 29 psig .
These have been either 70 series or 65 on minus 2 steel rims fitment.
I would never venture out and drive below -20F.
I could see in cold parts of Canada that if inflated at STP ( say, a garage) you would want the tires over placard
due to the 1psig/10deg drop. Then the tires would be 4 UNDER placard
This show the inadequacy of not specifying the ambient temp for a cold fill, and
the assumption of STP
conditions. A chart is really needed here.
Now here is a instance where it would be good NOT to read the O.M.