The US gallon is slightly less than a imperial gallon. The conversion is IMP x 1.201 = USG. So 5 imp gallons x 1.201 =6 usg. Or to go the other way, USG x 0.833 = IMP
Thanks for that question well this is what i found out
gallon: a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters Imperial units or the Imperial system is a...
One imperial gallon is approximately equal to 1.201 U.S. gallons. One imperial gallon is exactly 4.54609 litres, whereas one U.S. gallon is exactly 3.7854118 liters.