Gasoline prices are affected by a number of factors from supply and demand, taxes, politics, to the weather. Refineries buy crude oil based on the best available current price (which in turn is affected by a large number of factors on both the demand and supply side of the equation). Different types of gasoline such as those with higher octane number cost more to refine and therefore have a higher gas pump price. The final price displayed on the pumps usually includes taxes, which are set by the local or national government. In some countries taxes account for more than half the price. In the UK and much of Europe taxes make up about 70% of the total price charged at the pumps. By comparison, in the USA and Mexico this is closer to 20%. Exchange rates can also affect the price. Crude oil and its products are traded in US dollars. So if the British pound or Australian dollar rises or falls against the US dollar on the world currency exchanges, motorists in the UK and Australia will pay a little less or more for their fuel as a result.