You need to follow the following major things to get better mileage:
1) Use stock wheel size 165/70 R14, maintaining an air pressure of 34 PSI
2) Use full synthetic engine oil with API SN or ILSAC GF-5 with 5W30 viscosity.
3) Install original OEM air filter
4) Use high octane fuel
5) Carry out wheel alignment and balancing if required
6) Remove excess weight from vehicle.
This is other than what you have done.