What I would want to know is why was the speed you observed lower than that in the manual? Was it because of a mis-reading ASI? Mis-reading RPM guage? Aircraft overweight? General wear+tear on the aircraft causing a gradual degradation in its performance over its short service life?
How well do you know this aircraft? Does it normally fly at this speed, or did it slow down just for this flight?
I don't think there is a right or wrong answer to the question. The fuel consumption may well have increased by quite some margin when you increased your speed (do all new C172s have a fuel flow guage? Those I've flown all do) but you had plenty of fuel to cover this increase.
FFF
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