Fuel Consumption & The Tireless Mechanic

I recently acquired the Tireless Mechanic as an officer and the +5 Fuel Efficiency he offered certainly looked attractive. However, I seem to be using more fuel doing the run from London to Venderbight and back than I was with the Genial Magician assigned to be my Chief Engineer. I do know that before equipping the Mechanic, I could expect to use two barrels of fuel - maybe three - on that run. I’m now using considerably more - enough that I’m finding myself having to buy fuel at Venderbight to be sure that I won’t run out and find myself drifting in an area where I can’t even hope to be found by pirates and maybe get at least one barrel of fuel from the fight.

Is this a bug, or is there only so much one can expect if you’re still working with the starter engine?