6 km/lt is totally unacceptable in SX4 even (saying this since it is a big car and expected to have lesser mileage than a small car).
If you had been a too bad driver, how were you getting 12 km/lt earlier? Hence, 6 km/lt cannot be attributed to your driving habit.
Few things to check - My Alto milage got shooted up by 5 km/lt after these. Get these done from some other MASS necessarily the MASS haning good word of mouth in your locality since the MASS you went to didn't work for you.
1. Change the clutch
2. Change tyres and have tubeless tyres
3. No need for Alloy Wheels (they are too costly to establish the corressponding better mileage - that's what I think)
4. Have fresh engine oil, coolant, timing belt - you already have them
5. Engine overhaul - I didn't do it but may be good for you since your car is old
Last but not the least, if these don't solve your problem go to some other MASS necessarily the MASS haning good word of mouth in your locality, check for the reason.
If nothing works, I think it's time to sell off your beloved one since it's better to drive Honda City with better milage than an old M800.