I have a 2000 Santro, that did 6-9km/lit after overhauling and tuning (The car worked beautifully before CNG was fitted on to it). I changed everything on the car but wasn't able to get a good fuel average. Apart from bad economy, the car also hesitated on acceleration. In the end the issue turned out to be the cars's TPS and IDLE actuator. The mechanic who tuned the car had never calibrated the TPS after tuning work (because he didn't have an OBD reader). He also told me that my car's Idle actuator isn't working and placed a plastic shopper on my air intake pipe. Being the naive person that I am, I believed every single thing that he told me.
In the end I had to take the car to another electrician who had the right tools and an OBD reader, he calibrated everything and told me the IDLE actuator is working it just needed a little bit of calibration. The car works like a dream now and is doing 14Km/litre.
Meanwhile, I've lost all my trust in these crook mechanics here in Islamabad . All they know is to replace everything on the engine. What I learnt is that it's best to spend some time going through the Santro Factory manual (google it) and try to diagnose issues yourself. Then take the car to a mechanic and see what he tells you, if he's saying something different, you need to take it to a different mechanic.