mpg means miles per gallon. It is a way of talking about how far a car can go on a set amount of petrol. A high mpg number means the car is thrifty; a low number means it drinks a lot. This is mainly a US way of speaking, and you still see it in Britain on car ads. Though actually, a US gallon and a UK gallon are not the same size, so the same car can have two different mpg numbers, and the British one looks “better.” In Taiwan, and in most of the world, people talk about litres and kilometres instead — how many kilometres you get from one litre, or how many litres you need for 100 kilometres. You say it as letters: em-pee-jee. It is ordinary, a bit technical, not slang.