Trip computers can be funny. On my Sentra, fill-up the tank, reset the trip computer, and drive at a steady 50 km/h for a couple of kms and you'll get 3.9 l/100km. Get in a traffic jam for a couple of hours after that reset, and get 23 l/100km instead. Short trips and idling kill mpg. Averages for full-tanks are more relevant, and these days we have little traffic despite the cold and my manual Sentra delivers 8.2's, about a litre more than in summertime. Never had calculated averages higher than nines.