I've Honda City 2005 and I bought it brand new. On the very first day I drove it out of the Honda dealership, I immediatley went to the tyre shop and replaced the tyres from 165/80/R13 to 175/65/R14. Since then I've driven the car till now (Thats almost 5 yrs) and 127k KMs on the odo yet the mileage I get is around 13-15Km/Litre within city, while on the motorway, I get about 17-19Km/Litre when speed is maintained around 100-110KM/hr. (Both figures are w/ AC on full time)
Currently I've Michelen Energy Saver Tyres with 40K Kms done with the excellent road grip, soft enough with no noise yet hard enough to still have almost the same tread it had the very first day. I recommend you change the tyre size to the one I'm using as I've a few of my friends upgraded their City's tyres to the same size (175/65/R14) and still getting the same mileage.