I managed over 800miles on a tank completely by accident when moving house. It was almost all motorway mileage at around 70mph, half with a completely empty car towing my empty single axle trailer, the other half with the car and trailer stocked to the gills. The light came on at 750 miles, and I know I could have pushed to well over 800 if I didn't have to turn around and do another 150 miles!
I also managed to squeeze 72 litres into my tank once, and queried it with the garage owner. The pump was within allowable tolerance, apparently