Petrol is not a single chemical. It is a mixture of various hydrocarbons. Some of these hydrocarbons are fairly viscous whilst others are thinner than water. Petrol also contains aromatics which are normally gaseous at room temperature. By their nature, the viscous hydrocarbons will solidify before the aromatic hydrocarbons. Petrol has what is known as a "flash-point" of around -71.7 degrees Centigrade, give or take a few degrees depending on the exact composition of the petrol. At this temperature, petrol is still a liquid and this flash-point is the temperature at which the vapour of petrol will catch alight. Now, as you can imagine -71.7 degrees Centigrade is a very low temperature - but it gets worse. The more viscous hydrocarbons contained within petrol will become solid within a wide range of atmospheric temperatures. However, the aromatic hydrocarbons will not solidify until around -129 degrees Centigrade to around -185 degrees Centigrade - the composition of the petrol again plays a part in this. Sometimes miniscule amounts of water are present in petrol. This water will freeze-out at the same temperature as it does in the environment ie 0 degrees Fahrenheit.