Just to share, i recently managed to test my Kia Optima K5's FC at a constant 110kmph for almost the entire full tank. Based on the 'fill-drive-fill-measure' concept, i covered 1011km on 76L of fuel. 90% of the journey was at a constant 110kmph. This equates to about 7.52L/100km average over 1000+km.

My previous record was 780km on 80L of fuel driving at 160-170kmph on 80% highway. This equates to 10.2L/100km. I measured this when my car was about 2 months old.

Fyi, the K5 is considered slightly underpowered due to its higher body weight. So these FC figures are quite acceptable i guess.