Commodore V6 with sports pack - about 10 litres per 100km - about 2/3rd of driving is on highways.
Suzuki Swift - 6 litres per 100km - closer to 3/4 highway driving. Depends a lot on how much DH is being a leadfoot. When we had the Colt it was 5 litres.

DH has always been big on our cars having decent runs. Especially when we lived in Melbourne and did so much stop start driving. We always welcomed the big drive to NSW and back.