a bit of a stupid question but will be very helpfull if someone knew the answer.If not BMW thn any other car like it would be okay.Thankyou ! XoX

Try Google searching "l/100km bmw 518i"

You'll probably see some answers like 7.4 or something.

Then simply divide that 100 by 7.4 (or whatever the figure may be) and you'll have your answer.