Tata Nano — Poor mileage of 10.5 Km per litre with TATA NANO

There have been absolutely contradictory reviews about the mileage of the Tata Nano on portals like Cardheko and Mouthshut. While some Nano owners have written that they are getting an excellent mileage of 21 Km per litre in City Traffic with the AC on there are other Nano owners who have written that they are getting only 10 Km per litre or even lesser in City Traffic with the AC on.
How can there be such a wide variation of over 50% in fuel average whatever the driving conditions or driving skills may be ?
I am a Nano LX owner and my Nano is consistently giving just 10.5 Km per litre for the past nine months having covered 4000 Kms. I have carefully tested my Nano for the last six months running 2500 Kms and have with me irrefutable data to prove that my Nano is consistently giving a mileage of only 10.5 Km per litre in City Traffic. I have passed on this data to Tata Motors.
Mr. R Gopala Krishnan - Senior Manager - Customer Support, Tata Motors Chennai has driven my Nano along with a two member Plant Testing Team for three days and has orally confirmed that my Nano gives only 10.5 Kms in City Traffic. He has also confirmed that my Nano also gave him a mileage of 23 Km per litre when he tested it on the Highway which is a blatant lie because I have tested my Nano on the Highway with a diagnostics laptop attached and the mileage recorded was only 18 Km per litre.
From what I have gathered in the last six months speaking to several Nano owners in Chennai and Kolkatta is that most of the Nano’s are giving 16 to 18 + Kms per litre in City Traffic with the AC on and the mileage apparently goes as high as 21 Km per litre in City Traffic with the AC on which is really fantastic.
At the same time, there are a handful o[censored]nfortunate defective Nano owners like me who are consistently getting just 10 to 11 Km per litre in City Traffic with the AC on.
This kind of variation is absurd and what is even more absurd is the statement of Mr. V. Ramesh Kumar (another Senior Manager with Tata Motors, Chennai) wherein he has confirmed in writing that “Your car is absolutely trouble free and the fuel average you got in such conditions is very appreciable”. Absolute nonsense and pasteurised .
I am still waiting for Tata Motors to break their silence and come forward to do whatever needs to be done so that I also get a mile of 16 to 18 Km per litre.