The paper aims to shed some light on the impact of fuel prices and technological relatedness on green and non-green patenting dynamics and lock in to fossil fuel technologies. Specifically, we investigate whether green technology efforts come at the expense of other environmental or non-environmental invention activity. To do so, we employ Self-Organising Maps (SOMs) to detect the main technological domains exploited by the automotive industry during 1982–2008, using Triadic Patent Families (TPF) to proxy for the technological efforts in each technology field. The paper adds to the literature by examining explicitly whether fuel prices (used as a proxy for carbon tax) and technological proximity foster the substitution of non-green patents by green ones. In addition, we provide a novel contribution by testing whether these determinants impact on the competition among low-emitting vehicles. Our findings suggest that higher, tax-inclusive fuel prices are effective at redirecting patenting activity from non-green to green technology fields. Moreover, we observe that tax-inclusive fuel prices also induce a shift in patenting activity when we perform the analysis solely on green technology fields. Although this might result in potential lock-in to sub-optimal substituting technologies, our findings suggest that competition in the domain of environmental technology is focused mainly on ‘greening’ conventional cars and developing low-emission vehicles.