Poor mileage and stylish car

Car looks smashing and gives better mileage in highways. But in city limits it shows 8km/l, but it doesn't even give this much. Even the mileage reader shows wrong mileage. I took to service centre for mileage testing and they note down current mileage and filled the fuel to neck to neck and as per our calculation it came around 7.5 but the dashboard shows 13l/km, and the service centre guy started arguing that they believe the mileage shown in the dashboard. Driving experience is good but mileage is very poor.