Are you talking about fuel efficiency on CNG? well if you install CNG in any sedan car, i guess the fuel efficiencies will be more or less similar. Again as Junaid said, if that's the case, why dun you put CNG in your saloon?
And if you wanna know about best fuel efficiency on Petrol, then City Vario is the best choice, it's mileage on petrol is awesome. However, you're right, it's not compatible with CNG. So only buy a City Vario which has never been tampered with installation of CNG kit and just run it on petrol.
If you want to use CNG in any case, then get a civic auto and install CNG in it.