The Owner's Manual for the 2014 Mustang indicates that you should not drive the car with high performance tires and ambient temperatures below 40 F because of reduced traction.

It also warns against even moving the vehicle with ambient temperatures below 20 F because it could result in cracks in the tire. They recommend removing the tires at low temperatures.

My question is: How critical is this?

 

summer tires become very hard at cold temps, those numbers are obviously conservative but i wouldn’t go more than 5-10 deg past what they say

 

Yeah, the summer performance tires are just a bear in the winter, especially if you get caught in snow. I had that happen with my T/A once and the Goodyear GS-D3 tires (summer max performance) became rock hard and just no traction. It was hard to believe what happened. On the other hand, absolutely no issues with the Goodyear Eagle GT all-season performance tires on my Mustang. Great in everything!! They sure make the compounds different. You really have to adjust to whatever part of the country you live in, or own two sets of tires for the different seasons.