bhai, I see ur oriel is a gas guzzler... get some of the following things done to it and I hope mileage will improve very much...
mere chachoo paas oriel prosmatec hai and she does 16-18 km/L on highway wd AC.
in city wd full AC, about 12- 15km/L..
1- get its air filer changed
2- get its engine tuned up and fuel filters changed...
3- if needed, clean the fuel lines as well...
is ke baad ek full tank Hi octane ka use kijeye and ur mileage will become much much better inshAllah,...