This work studies the power loss in a real axle box gear transmission under no-load operating conditions. The influence of the rotational speed and of the lubricant viscosity was considered. An experimental strategy was adopted in order to measure the overall power loss of the axle box, as well as to isolate the power loss contribution of the major components: crown wheel, pinion, seals, and the tandem taper roller bearings supporting the pinion shaft and the crown assembly. Although it was not possible to directly measure the power loss generated by each component, the results allowed to identify the major contributions to the rear axle inefficiency. In particular, those from the crown wheel churning, from the lubricant squeezing due to the pocketing between the pinion and crown gear teeth, and from the friction in the preloaded tapered roller bearings of the pinion shaft and of the crown assembly. In average, the tapered roller bearings supporting the pinion shaft are responsible for 74% of the overall power loss of the axle box. A numerical model was developed to predict the power loss of the axle box and of its components, which was correlated with the experimental measurements with high accuracy (R2 = 0.9990). The combination of the experimental measurements with the model predictions allowed a full understanding of the axle box power loss behavior under no-load operating conditions. This analysis can be used for the design of an axle box with improved efficiency.