Depends on how you drive and how large your car is. My moms old Renault capture diesel uses around 5l/100km on average (combined city/highway) while she is driving and around 5.5-6l/100km while I'm driving. Just rented a Peugeout 208 1l diesel in Nice, France. Averaged 5l/100km during the week (500 km). Around 50/50 split regarding kms driven on highway and inside a city.