In article
..com, bob writes


One American gallon: 3.785 litres

One Imperial gallon: 4.546 litres

The difference stems not from the difference in gallons, but from the
two different definitions of the pint. The Imperial pint is 20 Fl Oz,
while the US pint is only 16. From that basis, the quart and the
gallon are each defined in the same way with respect to their relevant
pints.

And the original reason for *that* is that the British pint was
originally the space occupied by one pound of dried peas (God knows
why) whereas the US pint was defined as the space occupied by one
pound of water, which seemed to be a more accurately reproducible
quantity.
--
Bill Borland