Military vehicles experience a wide range of duty cycles depending on place and purpose of their deployments. Vehicle fuel consumption directly depends on those use cases, which are ranging from patrolling during peace keeping operations to direct engagements in hostiles areas. Vehicle design should accommodate this wide range of operation modes to maximize the vehicle practicality over their service life. This paper aims to quantify the sensitivity of the powerpack design for a notional 15-ton series-hybrid electric vehicle regarding different vehicle use cases including the highly dynamic military drive cycles. The optimal generator design for a powerpack (engine coupled to a generator) will be established for each of the use case separately by previously developed Finite-element based optimization routine and iteratively benchmarked by Dynamic programing. Then a newly developed energy management strategy will be used for a bias-free comparison among the optimized designs to quantify the fuel economy tradeoffs of selecting the best candidate for further development. A special care will be given to maintain constant maximum power rating to investigate the change of the machine design rather than a simple change in the e-machine size factor. The results can be used to further refine the methodology of the powerpack design and quantify its sensitivities on the vehicle fuel economy under various use conditions, rather than designing around a single drive cycle.