The growing need for sustainable urban development has intensified efforts to assess and improve mobility systems in European cities. Indicator-based approaches are pivotal, allowing for the quantification and comparison of mobility performance across diverse urban contexts. Composite indices, by integrating multiple sustainability dimensions, offer a structured framework for evaluating progress towards more sustainable transportation. This study contributes by adapting and applying a Sustainable Mobility Index (I_SUM) to assess mobility conditions in eighteen selected Italian and German cities. The research aims to analyse and compare how these cities are progressing with sustainable mobility measures. A quantitative methodology within a GIS environment is employed to compute the I_SUM. This composite index is based on multiple indicators grouped into eight domains, derived from existing research, to capture context-specific differences in sustainable mobility development between the Italian and German urban settings. The methodology involved: (1) assessing the availability of high-quality, open-source data and undertaking rigorous data collection and preparation for the selected cities; and (2) normalizing all indicators to ensure comparability. The findings, visualized through comparative charts and diagrams, provide insights into how these cities are advancing towards sustainable mobility, highlighting strengths and vulnerabilities within specific domains. This analysis aims to inform potential policy adjustments to enhance urban sustainability and address identified gaps in mobility infrastructure and policy.