2 U.S. Gallons Or 8L
According to the owners manual my 04 cooper has about 8 Litres of gas left when the low fuel light comes on. I also ran it below empty once and put 51L of gas in the 50L tank. Remember it has a filler tube as well that takes a couple of litres. I must have been running on fumes but did not run out of gas.
Communist dork? The United States is one of the few remaining nations in the world that actually still uses the Imperial system (technically they use 'United States customary system' but is almost identical to the Imperial system). Most have converted to Metric now...