having worked in a tire shop I can say that you should always go with the vehicles PSI number, not the one the tire shop filled them up to. That is unless the vehicle owners manuals states a higher PSI than the max on the tire, in that case use the max on the tire.
Our owners manual states to run 4.3psi (30kpa)higher than the summer number (38) So that is what you should do. Remember this is a cold number, not to be checked after you have been driving. Another thing to take in account is ambient air temp. As the temp drops, so will your air pressure. So when its minus 40 you need to add air. And dont check the pressure in your heated garage.
Also dont run the car with tire chains, or above 120km/h (75mph) with winter tires. Its all in your manual. That last one I dont follow as the Sotto Zeros are speed rated to 150mph
Hope this helps.
Here is a quote direct from the owners manual
Snow Tires
Use snow tires on all four wheels
Don't go faster than 120 km/h (75 mph)
while driving with snow tires. Inflate
snow tires 30 kPa (0.3 kgf/cm2, 4.3 psi)
more than recommended on the tire
pressure label (driver's door frame), but
never more than the maximum cold-tire
pressure shown on the tires.
Your vehicle is originally equipped with
summer tires designed for optimum
traction on wet and dry roads. If your
vehicle is to be used on snow and ice
covered roads, Mazda recommends that
you replace the tires originally equipped
on your vehicle with snow tires during the
winter months.
WARNING
Mixing Tire Types:
Using tires different in size or type is
dangerous. Your vehicle's handling
could be greatly affected and result in
an accident. Use only the same size
and type tires (snow, radial, or nonradial)
on all four wheels.
CAUTION
Check local regulations before using
studded tires.
qTire Chains
This vehicle cannot be operated with tire
chains because it could cause interference
with the vehicle body and scratching.
Before Driving Your Mazda