The U.S. Army is undergoing a major transformation to ensure that its future capabilities meet the needs of the nation. One element of its transformation strategy is the recapitalization (RECAP) program, which entails rebuilding and selectively upgrading 17 systems. The program has continuously evolved, with ongoing decisionmaking about what types of system modifications will occur and the scale of programs. Greater insight into aging effects can facilitate such decisions. This report describes regression analyses carried out to assess how vehicle age relates to annual repair costs and availability (or, conversely, downtime) for High-Mobility Multipurpose Wheeled Vehicles (HMMWVs). The study found that estimated repair costs and downtime increase with vehicle age, with the strength of both effects tapering off slightly for older vehicles. The study also found that vehicle usage, location, and variant type have significant effects. These estimated cost-versus-age and downtime-versus-age relationships were embedded in a vehicle replacement model that, given assumptions about the cost and effectiveness of RECAP, can be used to estimate the timing recommended for RECAP and replacement of HMMWVs. The model results indicate that the Army might be able to achieve modest life-cycle cost savings by replacing some HMMWV variants at an age lower than the current average fleet age, but the savings from RECAP will depend on the program's costs and its effectiveness in reducing future vehicle maintenance costs and downtime.