City Automatic works on A CVT principle this is Continously Variable Transmission. This basically means that unlike other automatics this does not have 3 or 4 speeds. The torque curve for this type of transmission is excellent and you will not feel any jerks as other automatics when they change gears on hard revving.
The basic difference between CVT and other automatics on engine/CNG performance just has to do with the ECU the engine computer since this is linked with a transmission ECU. Apparently the Vario ECU is not mapped to use CNG and therefore the engine drives the transmission too roughly. Hnece the transmission fails prematurely on CNG.
Manual City's are more suited to CNG since the transmission is not directly controlled by the ECU and you have control on the gear as well as the RPM. I have a manual city 2004 and it gives me 14 KM/l witn AC in the city and 17 KM/l without AC. I have not exacty checked the milage on CNG but if you need an approximation 60 liter tank gives about 180 KM without AC and 150 KM with AC.