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

The advertised number is based on a specific test condition

The EPA has two profiles, one for city and one for highway, that involve speeding up, coasting for a bit, slowing down, speeding up, etc over the test. The city test is about 20 minutes long and the highway one is 12.5 minutes long. The rated fuel economy is only true for those specific test profiles which is why people generally see different real world numbers

You can see the test profiles here

https://www.fueleconomy.gov/feg/fe_test_schedules.shtml