Hi,
Well I also have eagle shape prosmatic and its 2006 model and its giving me 12/13 within city with AC and 15/16 on the highway with AC .. When I bought it the guy just installed the CNG in it and used it for 1 year but I got that CNG removed and now its tuned on petrol only, engine is in very awesome shape as it has a very low millage when I bought it and was just like brand new, I got its timing belt changed, its gas kit changed, coolant changed, fuel filter changed, and also keep regularly changing its oil and air filter and its doing some amazing stuff for me now, average is awesome and pick is also great and it can reach easily upto 150 plus in a very quick time and I am very satisfied with it now still have to change its tires and the maximum that I took it was to 180 cuz I have old and dump tires will push its pedal more on motorway once I got the new tyres in it but the average should be 12/13 with AC within city if your prosmatic civic has all things 100% working and good or may be 2004 model has some change from 2006 which I dont think there is any ..