In what way does the mpg or kmpl number of a car depend on its speed? Is the advertised number based on average speed or a specific speed (say, 60 km/h)?

Question

In what way does the mpg or kmpl number of a car depend on its speed? Is the advertised number based on average speed or a specific speed (say, 60 km/h)?

In: Engineering

Answer 1

As a supplement to the other answers, fuel efficiency goes down as vehicles reach high speeds because they have to push harder through the air. Theoretically the force goes up with the square of speed, so a car that gets 40mpg at 50mph should get around 10mpg at 100mph. Engine efficiency and other sources of friction mean the difference won’t be so extreme.

Note that the metric unit for fuel economy is litres per hundred kilometres and low numbers mean better economy.