Per [1] the speed of light in a vaccum is exactly 299792458 m/s.

Per [1] the speed of light in a vaccum is exactly 299792458 m/s.
Per [2] a furlong is 220 yards and a yard is exactly 0.9144 m.
Per [3] a fortnight is 14 days.

As I didn't find a good authority for what definition of a day a fortnight is
measured in, I'll assume here a day is 86400 SI seconds.

Thus, the speed of light in a vaccum is approximately
1.80*10^12 furlongs per fortnight.

1.80*10^12 = 299792458*86400*14/(220*0.9144)

[1]	Resolution 1 of the 17th meeting of the CGPM (1983)
	http://www.bipm.org/en/CGPM/db/17/1/
[2]	Weights and Measures Act 1985
	http://www.legislation.gov.uk/ukpga/1985/72
[3]	The Concise Oxford Dictionary, 5th Edition, 1964, p. 480