One of the reasons petrol/gasoline is so useful is because it packs a lot of energy into a small space.
Specifically, about 36MJ per litre (that’s 36 million joules per litre).
To get a sense of what this feels like, think about how hard you work at the gym. You know when you’re on the machine and it tells you you’re working at 100 Watts? If you work at that rate, then in order to generate as much energy as is contained in 1 litre of petrol, you would have to stay on the machine for over 4 days solid (or, another way of looking at it shown below, is that it’s the same as 100 people being on the machine for 1 hour each)
Using that first calculation, if you drive a car with fuel efficiency of 40 Miles per Gallon for 50 miles, you’ll use 5.7 litres or roughly 205 million Joules.
Put this number back into the second calculation, and you’ll see how many days in the gym that amounts to.