Well, let me try and deal with the conversion here. 350km if you took it down to the gas warning light leaves about 3 gallons as a reserve which means you burned roughly 13 gallons (about 49 litres). So that means you're getting a little over 16 miles to the gallon, or 7km to the litre, or 26.9km to the gallon. If this is mostly city driving, it probably will not improve much, that is about right. If you're doing highway driving, then you can expect more, probably around 22 miles to the gallon, or 35.2km to the gallon, or 9.28km to the litre.
Phew!