In a sense every speed is a conversion speed, that is, a way to convert time into space and vice versa because multiplying a time interval (duration) times the speed of an object leads to the corresponding space interval (length). In some contexts, i.e., a transportation mode or physical medium, there is a particular speed, the conversion speed, that reflects the context in general and does not depend on the speed of any particular object in that context. This conversion speed applies to all objects in its context, except for the known speed of an object or signal.
In the context of high-speed jet travel, the speed of sound may be the typical speed of travel. Also, in the context of sound waves in air a standard speed reflecting ideal conditions may be the reference speed. In both these cases the conversion speed is the speed of sound, also known as Mach 1 because in that case the Mach number equals 1.
T. S. Shankara takes a related approach in his article, Tachyons via Supersonics (Foundations of Physics, Vol. 4., No. 1, 1974, p. 94-104). He draws a parallel between acoustic and electromagnetic waves, derives the Lorentz transformations in this way, and shows that the signal velocity is unrelated to its maximality. His goal is to suggest the possibility of tachyons — of which there is a large literature. This is consistent with what we have shown, too.