How cold does it have to be before diesel gels?

Driving in very cold temperatures is a non-issue, as fuel recirculates from engine back to tank.

And, those who say that fuel is "acclimatized" for current conditions are correct. BUT (large BUT) if you filled up in the summer or fall, that will not help if temperatures drop to the cloud point of the fuel in your tank.

Every Walmart has PS brand anti-gel. Put it in your summer grade diesel and drive/run diesel generator long enough for it to circulate.