Like electrical wires, axons carry signals from place to place. However, unlike wires, because of the electrochemical mechanisms for generating and propagating action potentials, the performance of an axon is strongly linked to the costs of its construction and operation. As a consequence, the architecture of brain wiring is biophysically constrained to trade off speed and energetic efficiency against volume. Because the biophysics of axonal conduction is well studied, this tradeoff is amenable to quantitative analysis. In this framework, an examination of axon tract composition can yield insights into neural circuit function in regard to energetics, processing speed, spike timing precision, and average rates of neural activity.