Intensification of groundwater irrigation is central to goals of improving food security and reducing chronic poverty faced by millions of rural households across the eastern Indo-Gangetic Plains (EIGP) of Nepal and parts of eastern India. At present, levels of groundwater use and access in the EIGP lag far behind other areas of South Asia despite abundant available groundwater resources. A key reason for prevailing access constraints is the dependence on diesel pumpsets for accessing groundwater, which are typically unsubsidised and therefore expensive to purchase and operate. To date, efforts to reduce access costs have focused almost exclusively on how to incentivise adoption of alternative electric or solar-powered pumping technologies, which are viewed as being cheaper to operate and less environmentally damaging due to their lower operational carbon emissions. In contrast, there has been little attention paid to identifying opportunities to make existing diesel pump systems more cost effective for farmers to operate in order to support adaptation to climate change and reduce poverty. In this study, we use evidence from 116 detailed in-situ pump tests along with interviews with pumpset dealers, mechanics and farmers in the Nepal Terai to assess how and why fuel efficiency and operational costs of diesel pump irrigation are affected by farmers’ pumpset selection decisions. We show that costs diesel pumpset irrigation can be reduced significantly by supporting and incentivising farmers (e.g., through equipment advisories, improved supply chains for maintenance services and spare parts) to invest in newer low-cost, portable and smaller horsepower pumpset designs that are more effectively matched to local operating conditions in the EIGP than older Indian manufactured engines that have historically been preferred by farmers in the region. Such interventions can help to unlock potential for intensified irrigation water use in the EIGP, contributing to goals of improving agricultural productivity and resilience to climate extremes while also strengthening farmers capacity to invest in emerging low-carbon pumping technologies.