Good Car overall, but poor mileage in petrol variant.

I purchased a Sail UVA LS ABS model from Kaysons Motors, Siliguri on 08/12/2012. I was the first customer of the petrol variant. Initially, I had booked an I20 Magna(Optional), but as i saw the new Sail model in ads and the net, i decided to go for the Sail UVA as it is less costly and gives more mileage than I20. But i am getting only 12 km/litre instead of the 18.20 as advertised for the petrol variant. After the first checkup in a month, the company people are telling me that i will be getting better mileage after 5000 kms. But i am sceptical about it. Overall, it is a good car in looks, space and comfort. Pickup is also good and if I can get 15 kms/litre in the highway, I will be satisfied. The front side looks good, but the back view is not up to the mark. Also if the power window controls were in the door itself, it would have been easier to perform the window functions while driving. As instead of the front window sometimes I press the rear window switch while driving. And the horn switches are not easy to operate while turning the steering.