Modularity in vehicle systems can reduce total lifecycle cost and improve adaptability for ground military fleets. This paper quantifies the adaptability of vehicle fleets to changes in supply route characteristics defined by route distance, threat level, and damage probability. Each characteristic is discretized into three levels, and the adaptability analysis is performed over 27 scenarios with significantly different characteristics. Fleet adaptability is measured in terms of two fleet-level metrics, namely the total fleet mass (which is correlated to the total cost) and the personnel requirements. Results show that modularity improves adaptability in terms of total fleet mass compared with a conventional approach, while providing a comparable adaptability in terms of personnel requirements. A sensitivity analysis is also performed on module characteristics of the modular fleet. This analysis shows that changes in relatively small parts of the system can change the values of performance metrics substantially.