55 is the volume of the cylinder and not weight. So its 55 liters and not kg.
55 liters means 55 liters of water can be filled into it.
weight of 1 liter of water = 1 kg
weight of 55 liter of water = 55 kg
weight of 1 liter of cng = 0.18 kg (approximately)
weight of 55 liter of cng = 55 x 0.18 kg = 9.9 kg (max pressure 200 bar)
Please note that these cylinders are not manufactured for pressures above 200 bar, so it is not safe to go for a full tank if the gauge at the cng station reads above 200 bar.
Please also note that :-
1 liter cng = 0.18 kg at very low temperatures. Normally at cng stations and especially at ones where there is alot of rush, the gas does not get much time to cool down so the weight per liter decreases to like e.g.
1 liter = 0.15 kg
So its normal if a 55 L of cylinder fills up with 8 kg.