Wasalam.
If you use Indus Highway its around 1200 km one way. but road is dual carriage way which will be extremely difficult to drive on such a long route. Now N-5 it will take 1800 km one way but road is ofcourse double. So, its your choice now. Regarding fuel economy you may have checked its fuel economy on highway. N-5 road in Punjab (Bahawalpur) till Peshawar is good but don't know about Sindh condition. You can also take motorway. But still I will not recommend to travel by car especially in your new Cultus. Its better to rent a car When you are in Peshawar if you need because what you save by coming in car maybe it will go away in wear & tear.