my vitz is in this thread, near the beginnign i think and the specs are also here. as for mialge, it usually gives me 13-15 in town, now has approx 89000 on the clock. as for your car, i'd suggest you get it checked but not necessarily changed.
right now the ECU isn't getting a signal for your injectors which is why you are getting the light. it must have een disconnected during the CNG setup to allow for switcfhing between fuels. an emulator will turn off when on petrol and turn on during CNG to tell your ECU that the injectors are ok even though they will be off during use of CNG. as far as i know you don't any other choice.
please send me the original tyre size of your city and i'll tell yuo what can be fit. as far as i know you'll have the same options as the vitz.
i always appreciate good things.