## 5 Discussion and Conclusions
### Limitations and Recommendations for Further Research  
This article deals with circular public tyre procurement based on the GT model with the elements causal condition, context, phenomenon, reaction strategies, and consequences. The element 'consequences' describes the desired public tyre procurement of the future with changed priorities and framework conditions and more sustainable tyre CPP which influences the market as a whole. Consequences can be analysed from short to medium- and long-term perspectives. As explained in the next paragraph, a period of five years provides appropriate framework conditions for next research steps. On this basis, a fundamental question regarding the future nature of the GT element 'consequences' is which characteristics the tyre market will have in five years. This question is linked with three sub-questions. The first sub-question concerns the degree of stakeholder engagement for sustainable tyres reached by 2026 and the drivers which will shape the character of this future market. In addition, one of our recommendations referred to standardisation, which means reaching agreements on particular specifications. An important second question is, therefore, which agreements will be reached to support CE tyres and whose interests were addressed the most. Therefore, this sustainability aspect also provides an interesting topic for standardisation research. The typical standardisation period of four years led also to our above-mentioned specification of an appropriate investigation period of short-term changes in the tyre market for all three sub-questions. The third sub-question is whether public procurers' purchasing power could be used to stimulate the development of sustainable tyres significantly and, if yes, which lessons can be used for other sustainable and/or innovative products. Regarding the market as a whole, the potential effects of standardisation, visible from the fourth year on, have to be considered specifically. Therefore, five years provide an appropriate frame for analysing changes in the market the first time. An appropriate analysis of market changes also requires a specification of suitable parameters and a diligent description of their current characteristics. An example for analysing the effect of green public procurement policies in a specific product area is provided by [61]. In addition, the research gaps regarding the Grounded Theory element 'consequences,' limitations also exist in a broader context. At the beginning, the seven tyre management options which can support the realisation of a CE according to [7] were presented. This article focused mainly on options 4 or 6. Another option is, e.g., 'Refuse via reducing vehicle ownership and using alternative modes of transport.' Indeed, some public managers consider this option. In particular, in rural areas, however, these options appear to be difficult. Nevertheless, more insight into successful implementations of these options seems to be useful. Therefore, this aspect is also a topic for further research. As we wrap up this paper, the World Climate Change Conference is taking place in Glasgow. It aims to substantially contribute to the global sustainability strategy and realize a lasting impact on future climate development. Progress in a wide range of socio-technical areas is desired. This progress can also further develop the framework conditions for sustainable tyres post-2021. Scholars from various disciplines such as economics and standards research are invited to explore the future of sustainable tyres and sustainable mobility as a whole in more detail. Conceptualization: S.W., Investigation: S.W., Methodology: S.W., Writing--original draft: S.W., Writing--review and editing: S.W., R.S., R.G.S. and S.H. All authors have read and agreed to the published version of the manuscript. We acknowledge the support by the German Federal Ministry of Education and Research (BMBF) (ConCirMy project, measure ReziProK, funding No.