I had Mira Custom and i ve travelled on it LHR-ISL motorway many times. The first time i took it on motorway, i tried very hard to be very gentle on the car in terms of pressing the pedal and also kept the ac on manual mode and keeps on switching it on and off and kept the speed between 110km and 130km. All this was done to ensure best fuel economy. I got my tank filled from DHA, Lahore and went to PAF AHQ, Islamabad, then to F-10 and Jinnah Super and on my way back, got the tank top-up from Kashmir Highway in 20 liters. I almost travelled 450KM in 20 liters of fuel and the average comes out to be almost 23 liters. In my subsequent trips, i always managed to get 20 plus average irrespctive of my driving style. The RPM oscilates between 4 & 5 at speed of 110km to 130km and fuel average calculator in the meter shows and average of 20 to 30km.
However, on local driving i used to get an average of 16-17km per liters.