There's a GNU 'units' program that can do all kinds of units conversions very easily. Just type the expression of what you have, and another expression of what you want, and it provides the scaling factor.
For example, translating the speed of light into furlongs/fortnight is just a matter of:
% units
2438 units, 71 prefixes, 32 nonlinear unitsYou have: 1 c You want: furlongs/fortnight * 1.8026175e+12 / 5.5474886e-13
See