Abstract Rotary Vane Expanders (RVE) are very suitable prime movers for ORC-based power units in on-the-road transportation sector. RVEs suffer volumetric efficiency deficits due to leakages which limit the overall expander efficiency and can vanish their intrinsic benefits with respect to the other prime movers. Making reference to a 2 kW Sliding RVE type (SRVE), the paper presents a theoretical and experimental contribution which goes deep into the effect of leakages inside the machine and aims to quantify their amount and effects on the expander performances. The results showed that the volumetric losses increase the mass flow rate aspirated by the machine if the intake pressure is kept constant. This increase favors a greater recovery from the hot source (up to 50%) but part of it bypasses the vanes, producing a volumetric loss. An interesting feature is that part of this additional mass is exchanged among vanes and this produces a beneficial effect on the indicated power (16.6% increase with respect the ideal case). The resulting knowledge further supported the effectiveness of dual intake expander technology which allows to theoretically reduce the leakages between adjacent vane up to 60–70% ensuring an improvement of expander efficiency.