I what to calculate the l/100km i know the fuel efficiency the coefficient of drag and the weight also I would like to calculate the top speed of the car
You need to work out how many watts of resistance there is, from tires and from air. Cd is not enough, you need CdA, which you can get from Cd * frontal area that basically is width multiplied by height, but you need to take account width without mirrors and ride height to be precise. Rolling resistance or rr varies greatly. Also you need to consider drivetrain efficiency, 12-25% of power is lost to heat in drivetrain, you need to figure out what is correct percentage for car in question, FWD is least, 4WD, 6WD with conventional automatic without locking TC etc are most. Also take note if power figure you have is from wheels or from flywheel. To get really accurate you need to know how much fuel is consumed for energy produced g/kWh which varies by engine load and rpm, can go from 250g/kWh to 450g/kWh with same engine, just changes in engine load and rpm changes that. So depending how accurate you want to be, it can get quite complex to get l/100km, but you can get rough estimate for steady speed by figuring out how many watts of air resistance and tire rolling resistance there is, then take account drivetrain losses and see how much power is needed. After that use your fuel efficiency to see how much fuel it really takes to produce enough watts of movement.
