Without knowing the specific fuel consumption curve for the engine , one cannot really say what the effect will be by altering the engine revs at the same road speed . If say at 70mph the engine is running at its optimum speed for fuel consumption, altering it either way will make it worse. If though it moves the engine speed to a more favourable part of the curve it will be better, but if it moves to a less favourable part it will be worse. One is also moving it about on the torque and power curve as well so you end up with multiple effects . I suggest moving it by 2.5 % is unlikely to show a noticeable effect. One is more likely to notice the ride changing.
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