From the top of my head, an English Gallon is something like 4.5 liters. Yet, an American Gallon is 3.78 liters or thereabouts. Why is there a difference between the two? One would think that the American Gallon would derive directly from the English, what with the British influences and all.
I don’t know what the 4.5 (l) gallon, but heard of metric system using countries making a gallon from of 4 (l) which would be close to 4.5 qts, I don’t think the metric gallon caught on. I have only heard of this metric gallon once and is hear-say. Mayby someone else can help out
I think the US system came about after the inch measure was standardized. A US liquid gallon is exactly 231 cubic inches but none of the imperial measures correspond to an even number of cubic inches. Also the US has different volumes for liquid and dry (which is also different than imperial, but closer than US liquid) but the imperial system uses the same volume per unit for both liquid and dry.
Personally I find it frustrating in the kitchen that I have to use different measuring cups for flour and milk.
Well, I think we all agree that the metric system makes the most sense, and it would indeed simplify kitchen work, and whatnot 
I was watching Jeremy Clarkson’s Car Years on the BBC tonight, and noticed him doing some calculating in order to get the MPG for a Range Rover. Since the fuel station used liters, he converted it to gallons by dividing by 4.56, if I’m correct. I don’t think this was a “metric gallon”, because he specifically referred to liters as “Euro Nonsense” 
Any Limeys in the house?
Padeye explained the US Gallon superbly. Now we need to know what the English Gallon comes from.
A proper Imperial gallon is four quarts. So is a U.S. gallon, but they got the quart wrong.
An Imperial quart is 40 fluid ounces, but the U.S. version is only 32.
(Maybe it’s based on the same sort of reasoning that left the ‘u’ out of words like ‘honour’ and ‘colour’. )
And that means the American pint if BEER is 16oz compared with the UK pint at 20. Then there’s the whole froth issue.
I quite like the American size because it’s more manageable - just means you have to catch the waitress’s eye a little more frequently.
Someone–who shall go unnamed–asked a question similar to this a while back. Here is that thread:
http://boards.straightdope.com/sdmb/showthread.php?threadid=8953
Unrelated to the OP, some people believe that today’s vehicles are more fuel-efficient that those of yesteryear.
Not true–I understand that the Spaniards got several thousand miles per galleon.
They come from different sources–the Brits split up a barrel into a sensible number of parts, and we used a sensible number of a smaller measure (or something like that). I’ll look it up and clarify in the morning.
here’s your answer. the ultimate site on units:
http://www.unc.edu/~rowlett/units/dictG.html
The reason the Imperial gallon is different was that in the early 19th century (1828, I think), the British adopted a new definition of the gallon. The Imperial gallon is defined as the volume of 10 pounds of water at 62[sup]o[/sup] F. My impression is that this is somewhat in imitation of the definition of the metric system, at least the factor of 10 part.
What this means is that they screwed up a perfectly rational binary system that us Americans had the good sense to keep.
I don’t know if we can call the American Gallon perfectly rational, but OK - traditions are wirth something as well, I guess.
Just typical for the Brits: they try to immitate the metric system, but base it on the wrong temperature scale
Thanks for the answers guys!
What I meant was that the ratios between units in the US fluid measurement system are all powers of two:
1 gallon = 2 pottles[sup]1[/sup]
1 pottle = 2 quarts
1 quart = 2 pints
1 pint = 2 cups
1 cup = 2 gills
1 gill = 4 fluidounces
1 fluidounce = 8 fluidrams
See, perfectly rational[sup]2[/sup]. The Imperial system throws a factor of 5 in there to monkeywrench things (also their fluidounce is slightly smaller than ours, but that’s a minor factor).
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[sup]1[/sup] I like that word, pottle pottle pottle. Try saying it without smiling.
[sup]2[/sup] Well, almost. I deliberately left out the minim which is 1/60 of a fluidram. Ignore it, it just ruins everything…
Ah, rational as in making use of a constant ratio. Sure, it’s rational. Never knew this would turn into a semantics debate though