The liquid being measured determines the weight of 55 gallons. Gallons are used to quantify volume rather than weight. The density of the liquid being measured must be known in order to determine the weight of a given volume.
Water, for instance, has a density of 1 kilogramme per litre. 55 gallons are equal to 220 litres since one gallon is equal to four litres. Multiplying the volume of the water by its density yields the weight of 55 gallons of water: 220 litres of water multiplied by 1 kilogramme per litre yields 220 kilos. The equivalent weight in the imperial system is roughly 485 pounds.