@9bl.
thanks for sharing your experience and formula. For now that's the something that can give an IDEA of the real millage.
But for some percentage of cars that formula would be seriously giving flawed data. For example a friend of mine owns a mehran and he lives about 3 kilometers from the his Office, Pakistan Cricket Board.
He does 6 kms a day excluding sundays and saturdays. He has no family living with him. He has all his meals from office. His gym is in the office. He shops for groceries from a store next to his house. He rarely goes out with friends because he spends most of his time in PCB. For over 5 months of the year he is on tours with the a team or under 19 team or with the national team.
Even after 6 years his car has done less than 10K. What's funny is that he has a colleague with the exact same job, his car has done just about 12K in 5 years, he owns a chevy 800cc. He has similar life style.
I urge people to go for 9BL's formula but add more variables in your calculations like, distance from office, lifestyle, etc etc.
Hope that helps.