It's a common misconception that the U.S. uses Imperial Units. Sure, we use the same unit names as Imperial, but really, ours are just a bit off, except for liquid measures, which are off by a fairly large amount.
Modern U.S. standard units are also set to be fixed values of the metric system:
1 pound-mass = 0.45359237 kg (exactly)
1 inch (US Standard)= 2.54 cm (exactly)
1 gallon (US liquid) = 231 cubic inches (exactly) = 3.785 L (approx.)
But, compare:
1 gallon (UK "Imperial") = 4.54609 L (exactly)
There's also the quirk that, for land surveys, the older definition of 1 foot = (1200/3937) meter is still used. The difference between the standard foot and the survey foot is at the fifth decimal place, so most of the time, it's an insignificant difference.