The original / factory recommended size is 175/65R15. The oem wheels are 5.5" wide. Using bigger tyre does affect fuel economy, specially more, if you use a heavier foot. Also speedometer and odometer will tell you less speed and less distance covered (by approx 4.4%, i.e 4.4km @ 100km).
Unfortunately, the oem size of 175/65R15 is locally available only in Eurostar tyre (made for factory spec of honda city). Many people swear by eurostars, and I suppose during routine careful driving they should do a decent enough job. However if one chooses to go for imported tyre, the choice is to go for bigger size. The size closest are 185/65R15 or 195/60R15. The 195/60R15 size is not freely available and not a regular import. 195/65R15 is freely avaialble, coz its the factory size for both honda civic and toyota corolla, hence availble in wide variety, always in stock and cheaper! Thats why one sees most people end up using the 195/65R15 on their Honda City, the larger size does mean a bit more comfort at the price of slightly reduced fuel economy and power. The 185/65R15 are available, but less variety in comparison.
Anyway, despite the above, ur mileage is still on the lower side. I have myself changed tyres on the Fit to Michelin 185/65R15 size. So far I have driven around 250km on the new tyres and fuel consumption seems only marginally affected. Previously around 20kmpl, now 19kmpl. Will update again if it changes with more driving.