Actually the legal definition of a gallon is 231 cubic inches, and has been for over 200 years. The number Suzie quotes is the metric equivalent of a gallon, however that is NOT how it is defined.
Actually, that was in the PAST, This is NOW.
The definitions have changed.
We ARE on the metric system for DEFINING ALL "English" units, and yes, you do have to make the conversion. I will find the explanation from NIST--National Institute for Standards and Technology-- and add it to this post when I find it a little later.
Probably only Ken will have the interest and patience to go through this.
Here it is:
This one explains the metric nature of the "English" units. You only need to watch the first minute and a half---https://www.youtube.com/watch?v=SmSJXC6_qQ8
Here is the text from the NIST Publication for the DEFINITION of NOT AN EQUIVALENCY to the pound:
Handbook 44 – 2016 Appendix B –Units and Systems of Measurement
3.2 Standards of Mass.
– The primary standard of mass for this country is United States Prototype Kilogram 20, which is a platinum-iridium cylinder kept at NIST. We know the value of this mass standard in terms of the International Prototype Kilogram, a platinum-iridium standard which is kept at the International Bureau of Weights and Measures.
In Colonial Times the British standards were considered the primary standards of theUnited States. Later, the U.S. avoirdupois pound was defined in terms of the Troy Pound of the Mint, which is a brass standard kept at the United States Mint in Philadelphia. In 1911, the Troy Pound of the Mint was superseded, for coinage purposes, by the Troy Pound of the Institute.
The avoirdupois pound is defined in terms of the kilogram by the relation:
1 avoirdupois pound = 0.453 592 37 kilogram
You see the pound is DEFINED by the metric measure.
The same thing goes for volumetric capacities like the gallon. I am still looking for the text in their paper. It may be in another paper that I cannot find right now. The gallon has been removed from the list of primary units of measure as of 1994-12-31 and it is DEFINED in terms of the SI as 3.785411784 liters EXACTLY.
https://www.youtube.com/watch?v=Oo0jm1PPRuo --this is for added interest.