The best mileage you get is when you move in the highest gear at a constant speed (I think for Mehran, 60~70 in 4th gear will give you best mileage).
Try to shift the gears at around 2200~2500 rpm. Avoid sudden acceleration and too much braking.
If these don't help, try to diagnose by connecting the ECU with a scanner to check if all sensors etc are working ok.