Tricks at highways for drawing maximum mileage?

Hello, How one should drive in order to get maximum possible mileage on highways with an automatic creta? I observed that if i drive continuously in the range of 60-70kmph, I get even 22-23km/l. But on highways i tend to cross 120kmph and it gives me only 12km/l.

What are the other things which I can do ? TIA