I have a Vitz 2003...
My choice: 14' Rim and 185/65 Tyre.
In this combination:
1st of all the Height of the car will increase saving from the speed breakers,
2nd is Good comfortable/Smooth drive,
3rd In such shift you will feel some decelaration but my economy with this is 12-13 Km/Liter of Petrol.
Best of Luck...