1 imperial gallon = 4.54 litres

1 imperial gallon = 8 pints, each pint being 20 fluid ounces (where one "fluid ounce" is the volume of water that weighs one ounce)

1 US gallon = 8 US pints , but a US pint = 16 fluid ounces so interestingly a US pint of water weighs one pound!

So...

1 US gallon = 16/20 of an imperial gallon (= 0.8 imperial gallon) - beware this when comparing US "mpg" figures

and 1 US gallon = 16/20 x 4.54 = 3.63 litres

Morgan are correct that a 12 (imperial) gallon tank is (near enough) 55 litres. Only I don't believe mine holds that much!

Much easier IMO to stick to metric units (ducks and runs for cover)
Nigel

PS who knows which engineering items are commonly, indeed almost always, specified in "inches" (or "pouces"), worldwide?