Cars Q&A

Recently, I have owned 3 years used Mahindra Scorpio Slx (CRDe) Model which is runned appx. 1,25,000 Km with Engine in Good condition. After taking it, I have replaced all four tyres and used Nitrogen Gas instead of normal Air in tyres. I have replaced Engine Oil, Gear Oil, Clutch cable, Air Filter, Diesel Filter. I run vehicle on highway with constant speed of 80 Kmph with minimum use of Clutch and Brakes. But I'm getting mileage of 10.91 Kmpl, which I think is too less. I expect it to be 14 Kmpl. What are the probable reason for that??