You can only calculate the highway/motorway average accurately because in city driving circumstances are never the same. Well! these are the fuel averages on Cuore 07 both on Petrol and CNG w/o AC at 100 kms/hour on motorway.
ON PETROL 710 kms in a tank of 32 litres average= 22.30 km/litre
ON CNG 331 km/50 litre CNG cylinder (My best ever)
Normaly it does 275-280 km/50 litre cylinder.
In City driving it gives 200-220 km in 50 litre GAS cylinder. on Petrol it is usually 16-17 kms per litre but again these are never the same.