Asalam-o-Alikum, Talha i bought Honda Civic 2003 VTI Orial non prosmatic arround 4-5 months ago. CNG was removed from it and now working on Patrol only. Engine is well tuned and done 160k on odometer. I used to drive arround 80-100KMH normaly on as low as 2000-3000 rpm on all gear shifts ( as its manual transmission ). on full fuel tank which is around 42-45 Litters i am getting 580-600 km with random AC which is arround 13.5-14KM/L. I do have Corrola 2014 1.3 old shape which give me same average as well. I am happy that i am getting this much average on 15 years old car.
When i get this car it was giving me 9-10km/L but after some tuning like engine Tapit adjustment, throttle body cleaning, spark plug cleaning, air intake cleaning, wheels balancing, wheel alignment & other sensors cleaning.
Honda Civic EXI have a very bad fuel average but VTECH Engine is so much powerful and Fuel efficient on lower rpms.
My suggestion for you is to tune up this car first. if still not happy with engine and mileage then sale it as it will cost you more for new engine.