Grass biomethane is a sustainable transport biofuel. It can meet the 60% greenhouse gas saving requirements (as compared to the replaced fossil fuel) specified in the EU Renewable Energy Directive, if allowance is made for carbon sequestration, green electricity is used and the vehicle is optimized for gaseous biomethane. The issue in this paper is the effect of the digester type on the overall emissions savings. Examining three digestion configurations; dry continuous (DCAD), wet continuous (WCAD), and a two phase system (SLBR-UASB), it was found that the reactor type can result in a variation of 15% in emissions savings. The system that as modeled produced most biogas, and fuelled a vehicle most distance, the two phase system (SLBR-UASB), was the least sustainable due to biogas losses in the dry batch step. The system as modeled which produced the least biogas (DCAD) was the most sustainable as the parasitic demands on the system were least. Optimal reactor design for sustainability criteria should maximize biogas production, while minimizing biogas losses and parasitic demands.