At the end of the day I thought the question was not "why was my speed wrong?", but "Was I wrong to increase speed?"
That's very true, Foxmoth, but the point I was trying to make in asking the question "why was my speed wrong" is that the answer to the second question almost certainly depends on the answer to the first.
I'm still not quite sure what the "correct" answer to the original question is. I can think of a few scenarios (i.e. a few answers to the question of why the airspeed was wrong) with definite answers. For example, if the aircraft is known to have an RPM guage which misreads slightly, then there is no doubt at all that it would be appropriate to increase the power to get the desired speed. On the other hand, if the aircraft was a little older and had simply become less efficient with time, then to increase the power above the POH manual would quite likely result in a disproportionate increase in fuel consumption - most likely not a major issue if Check Airman had full tanks of fuel for a short trip, but probably not the most appropriate course of action to use on a regular basis.
Without knowing what the reason for the reduced speed, all I can say with absolute certainty is that maintaining the slower speed would be the conservative option, but increasing the speed to the planned speed is unlikely to cause any problems if you can take the increased fuel consumption, and saves the small hastle of altering your ETAs on your plan. Personally, if I have an unexplained problem I'd probably go for the conservative option, but the problem as Check Airman describes it is so minor that being conservative is possibly over-cautious.
FFF
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