You really need to look after your car’s waterworks.
Believe it or not, modern cars don’t give much better fuel economy than older ones. Even back in the 1950s, specially tuned Morris Minors were driving 100 miles on a single gallon of petrol (that’s around 2.8 litres per 100 kilometres) and reaching speeds of 180km/h. The catch was, they could never do 180km and 100 miles per gallon at the same time and they couldn’t do either in everyday driving conditions.