Miller's Elements of Chemistry is in three volumes, Part 3 of which deals with Organic Chemistry. What I love about Miller is his detailed and practical description of the aspects of his world in 1856, and for example he goes into great precision on the subject of coal and coal gasification. He tells us that for illuminating gas Scottish coal is best, since it contains higher hydrocarbons [e.g. in modern parlance that would be ethene (ethylene), ethane, propane etc.] which burn to give carbon particles and emit light from the flame. He says that coal from Durham - Newcastle Coal - is the best for making town gas to be burned as a fuel. The expression "bringing coals to Newcastle" is a reference to the huge production of coal that went on there at one time, and that it is superfluous to bring more since no one would want it, hence a metaphor for any unworkable or unmarketable enterprise.
Miller quotes that one ton of Newcastle coal yields 9,250 cubic feet of gas and 13 cwt of coke. One cwt is a "hundredweight" of which there were 20 to the ton and so the quantity of coke amounts to 65% of the initial charge of coal into the retorts.
There are 35.3 cubic feet (cu. ft.) to the cubic meter (m^3) and so that ton of coal yields 9,250/35.3 = 262.0 m^3 of a gas with a "specific gravity" of 0.410. The latter is referenced to air and so if one m^3 of air has a mass of 1.20 kg, the equivalent volume of coal-gas has a mass of 0.410 x 1.20 = 0.492 kg. Multiplied by 292.0 m^3 this gives a total mass of 128.92 kg.
The Imperial "long ton" amounts to 2240 pounds (lbs) or 1016.05 kg (or 1.01605 tonnes). Thus the mass gas yield is 128.92/1016.05 = 12.7%.
The difference between 65% coke plus 12.7% gas and the initial ton of coal leaves 22.3% which we may attribute to coal tar.
I noted in the previous posting that one m^3 of the gas of the composition quoted there has a mass of 536.69 kg, and if a similar quantity of it were produced by retorting one ton of some coal, this amounts to a similar 262.0 x 0.53669 kg/1016.05 kg = 13.8%. The somewhat (9%) higher mass of the gas in the latter case may be due to the presence of more higher hydrocarbons, which will increase its calorific value slightly, or to carbon dioxide which will reduce it.
"Miller's Elements of Chemistry", Part III. Organic Chemistry", John W. Parker and Son., London, 1855.