Fuel economy and fuel consumption are upside down from each other: the farther a car goes on its fuel, the less fuel it needs for a set distance. So instead of multiplying, divide 100 by your number. For example, 15 km/L is about 6.667 L/100 km.
Divide 100 by your number100 ÷ 2.76 = 36.23 L/100 km