Hope this doesn't sound like too daft a question... but I'm English and a noob so please humour me :)
When I am reading recipes, brewing instructions, calculations, etc. from American documentation and they are referring to "gallons" are these imperial gallons or US gallons?
To me, 1 gallon = 4.5 litres. Google tells me that 1 US gallon = 3.8 litres. Quite a difference. Clearly, since we both split gallons into the same number of quarts and pints, this would affect the measurement of those too.
Although this affects pretty much everything I need to learn, I am asking specifically because of point 14 at the bottom of Palmer's page here http://www.howtobrew.com/section3/chapter18-6.html - using US gallons in this calculation gives my first every AG brew an extraction efficiency factor of 30 (yay!) whereas using imperial gallons gives me 25 (boo!).