I'm going to semi-hijack this thread and postulate that for Jefferson's system (in which the unit of length was a pendulum with a period of one second located at the latitude of Monticello, I believe) to catch on, it would have had to happen in a non-revolutionary North America where he could devote considerable time and energy into promulgating this new system with Franklin's assistance/prestige.
Let's assume, just for the heck of it, that there was no revolution and that a peaceful accommodation with home rule was accomplished (never mind the details here). That would leave Jefferson as some sort of higher level functionary analogous to secretary of state plenty of time to develop his system. Franklin's scientific and social connections could have led to its presentation at the Royal Society in London in the late 1770s/early 1780s, with Priestly and Watt (among others) as converts/proponents. As Great Britain and the Commonwealth of North America (for lack of a better name) industrialized, Jefferson's system proved to work well for design and construction; the system also meshed well with the decimal currency in use in the Commonwealth.
By the time steam-powered railroads became practical in the 1820s, the Jefferson system was well on its way to full adoption in the CNA, Great Britain, and its dominions/territories. At the same time, the French Civil War (1789-1796) and the subsequent victory of the Intellectuals spawned a move to purge France of ties to the old regime as thoroughly as possible, including the old system of measurements: that led to the development of the système française, based on the metre, defined as one ten-millionth the distance between the equator and the north pole at the longitude of Paris. With the marching of the armies of Napoleon, the système française spread throughout the continent into Russia.
The two competing systems stood side by side as technology developed in the 19th century, and the establishment of parallel systems spread into other areas as well: for example, the English-speaking world today uses the North American domestic electrical standard of 60Hz/120VAC power while the European continent and various overseas dominions use the continental system based on 50Hz/240 VAC. Temperature, however, seems to be the one area where the systems work in concert: the world uses the Rankine-Gibbs scale for thermodynamics, physical chemistry, etc. (absolute zero = 0°R; the triple point of pure water is approximately 492°R; the boiling point of pure water at sea level is approximately 672°R), and domestic use (e.g., weather forecasts) use the Fahrenheit scale.
With the increasing prominence of the CNA and the British Commonwealth of Nations in science and industry, the système française seems to be fading slowly: the Bundeskönigreich Deutschland (Federal Kingdom of Germany) formally adopted the Jefferson system in 1976, as did the kingdoms of Denmark, Sweden/Norway, and the Netherlands. Other European nations have followed suit over time; it is expected, however, that the Francophone world will continue to adhere to the système française, particularly since l'Academie Française has been charged with its preservation, along with its original charge of preservation of the purity of the French language.