Hehe... I guess I am the old man here..
I dont speed on highway, just cruising at 110 - 120 kmph. enjoying the scenery and of course save some money on speeding ticket. My wife also say she can fall asleep in the car... I usually get about 17 km/L on highways that means a full tank can go for about 1000+ km.
A tip to those who wants to get the best of their mileage;
1. Go easy on the accelerator. If you lanyak, sure koyak one. look at the consumption meter.
2. Plan your driving ahead. e.g. let the car slow down by itself if there is a moron blocking your path and slowly bring the car back to the desired speed.
3. Set your A/C at 18C. That way you really "cool down". :aetsch: hehehe..
On urban road, I get about 9-10 km/L. That means on full tank can go about 500-600 km.
It is a 2006 525. Please pardon a old man driving like a "snail" like me. Dont honk me next time I am in front of you. Otherwise, the auto finger will "finger" you. hahaha...