You said you will be driving mostly on NH which has less traffic (?). If you can maintain a constant speed of 60-80 kmph, you can get a max mileage of 16-17kmpl. I have seen 17 kmpl on Bangalore-mysore highway (150kms). I have observed that maintaining lesser speed in 5th gear doesnt help on the FE front. This is my experience, thare can be someone with a different experience/view. Though the car can easily touch speeds of 120-150+, do that only when you really need it, especially in the initial period. And, make it a paractice to open the bonnet once a week and check the fluid levels. And it is my guarantee that other than the soaring petrol prices, nothing else will prick you!