Excerpt from the manual
"Tank capacity: approx. 14.5 gallons/ 55 liters, of which approx. 2.1 gallons/ 8 liters are reserve capacity.When indicator lamp begins to stay on continuously, there are still approx. 2.1 gallons / 8 liters of fuel in the fuel tank.
It is important that you refuel when the cruising range falls below approx. 30 miles/50 km. Otherwise, the engine cannot be guaranteed to operate properly and damage may result.
Fill the fuel tank before it is completely empty. Driving to the last drop of fuel can prevent the engine from operating properly and result in damage."
I"ve tried driving with the low fuel warning lights on until the range to go shows 85km then refuel, only managed 47liters.
That is after the second click of the pump even though the manual says its full after the first click.
Read somewhere someone tried until the range to go shows dashes and he still managed to squeezed a few more miles out of it. BMW recommended RON 98 for best performance and better mileage. Maybe someone out there can verify that.