The graph is showing that at the constant revs needed to go at 50kmh, you will need a specific air flow to get the torque required. This air flow will give a specific air pressure in the inlet manifold, and hence a specific efficiency. Cruising at 50kmh on the same incline cannot produce different intake pressures. The graph is showing how if you change the load on the engine, the pressure will change due to the change in air intake and hence a change in efficiency. —
HandyHowie 1 hour ago