First of all you should find a driving cycle for your country or your city. For example New York driving cycle uses for big city. Then you should calculate BSFC in 750 to 900 rpm.In these speeds BSFC for a four stroke 4 cylinders engine with 2 liters capacity is about 410 to 420 g/kW-h, but I suggest you to calculate BSFC for common cars in your area.
As mr. hasanpour said you should choose a driving cycle. read https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/4247/ppr-354.pdf
then i suggest you use advisor to simulate your car, you can use your own coding BSFC depends on engine rpm. BSFC of a standing car depends on your car. if you use advisor you can see different figures of BSFC to RPM for its engines.