I will give it a try..

470km = 55 Liters of RON 98 gas

100km distance comes to 11.70 Liters (Real measurements)

My OBC shows: 9.7 L/100

Therefore my correction factor should be 1 - [(9.7-11.7)/11.7] = 1.17

Done in US friendly units:

1 liters = 0.264172052 US gallons

Therefore: 11.70L = 3 US Gallons / 62.5 miles

Comes to 20.8mpg average (aggressive driving + ac always on)