Modern automotive manufacturing and after-sale service environments require tailoring of configuration values or “calibrations” within the vehicle’s various electronic control units (ECUs) to that vehicle’s specific option content. Historically, ECU hardware and software limitations have led designers to implement calibratable values using opaque binary blocks tied directly to ECU internal software data structures. Such coupling between calibration data files and ECU software limits traceability and reuse across different software versions and ECU variants. However, more and more automotive ECUs are featuring fast microprocessors, large memories, and preemptive, multi-tasking operating systems that open opportunities to object-oriented approaches. This paper presents the CalDef system for automotive ECU calibration software architecture. CalDef uses XML database methods to define, develop, and deliver calibration values as objects in both manufacturing and service environments. To reduce consumption of precious manufacturing network programming time, CalDef defines overlays, called “cal overrides”, wherein only values that have been changed from their design-time defaults must be programmed into the ECU at “end-of-line”. This strategy limits consumption of programming network bandwidth and allows CalDef to be deployed in existing automotive manufacturing plant environments without programming tool changes. The first section of this paper traces history of automotive calibrations as motivation for CalDef followed by a discussion of legacy method disadvantages and CalDef advantages. The System Architecture section presents the detailed design followed by provide a brief overview of CalDef’s custom CASE tool suite. The last section contains a simple XML file example to illustrate the overall architecture.