I havnt owned or kept a Cultus ever, But Alto manages 10 km/L i have had my uncles car for 15 days, I was traveling from Defence to Tower via Mai kolachi and back home same route, The car wasnt in prestine condition but managed 10 which to me is okay.
Maybe cars tuned for CNG have a badder fuel average on Petrol, BTW Alto can differ upto rs.50,000 for being with and without FF CNG