[chris (OP)]

How cold must it become before diesel and petrol begin to freeze?

[Geezer]

Without knowing the composition of the particular fuels, it's not possible to give a very precise answer. However, petrol (gasoline) has a much lower freezing point than diesel fuel. The range for petrol seems to be somewhere between -40°C and -60°C, so it's unlikely you will ever have a problem with your fuel freezing if you run your car on petrol.

There are also a lot of different types of diesel fuel, but the "pour point" might fall in a range between -6°C and -18°C (*), although these values can be lowered with additives. The pour point is the temperature at which diesel turns into a sort of jell which can no longer be pumped, so it's essentially useless as an automotive fuel at, or below, those temperatures.

(* Marks Standard Handbook for Mechanical Engineers)

[Don_1]

As I recall, the 'waxing' temperature of winter diesel in the UK is -15^oC. But as Geezer pointed out, addatives can be used to lower the waxing temperature to around -22^oC.

Diesel is usually refined to be suitable for the region and season it is to be used in.

[chris (OP)]

Why do the two different fuel types, petrol and diesel, have different freezing points in the first place?

[alancalverd]

Because they are substantially different molecules. Diesel consists mostly of straight chains of 12 - 20 carbon atoms, like paraffin wax and poyethylene, whereas petrol is a mixture of shorter chains around C₈ and C₆  rings.