1. Fill up tank to full.
2. Reset your tripmeter - Google/ask how to use a tripmeter if you don't know.
3. Drive till tank almost empty or light flashes.
4. Fill up tank to full. This is VERY IMPORTANT - Note down the amount of petrol (in litres) used from pump readout or your receipt.
5. VERY IMPORTANT - note down your distance (in KM) travelled on the tripmeter, then reset it.
6. Do your maths. KM travelled / amount of petrol used = fuel efficiency (KM/litre).

All the agak-agak methods are useless with people citing different fuel economy using imprecise methods.
300km for full tank is odd, even my 15 year 1.3 Wira (M) gets 11-13KM/litre typical mixed-mode driving.