This paper investigates options for light-weighting truck trailers through a combination of material selection and structural optimisation. Critical chassis design load cases were established, and a parametric finite element model of a typical European-style 13.5 m long truck trailer built from steel I-beams was developed. The model has been used to show that existing longitudinal steel I-beams could be reduced in weight by 28% (140 kg) through shape optimisation alone. The model was expanded to analyse holistic composite trailer structures. It showed that up to 67% (1,326 kg) of weight could be saved by executing shape and material optimisation in unison. The approach highlights that design through parametric analysis allows for many different structural configurations to be assessed in terms of both mechanical performance and material cost. This facilitates the construction of a theoretical design space of a lightweight chassis, clarifying the weight reduction limits that could be achieved with lightweight materials and structural optimisation. The lightweight trailer chassis designs proposed here are also compared against a portfolio of shorter-term strategies for trailer light-weighting. These strategies are poised to have an increasingly important role in reducing the greenhouse gas emissions of the road freight industry.