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Since we are dealing with engines with high specific power, we can assume they are a little cammy with quite some overlap which means they get more efficient at higher RPM.
usually an engine is consuming the least fuel a tad under peak torque with WOT.
I bet your peak torque is 8K and peak power 9K.
What I get from your testing is that there is an optimal compromise of gearing for every size of engine and that longer gearing is not always the best compromise when dealing with smaller engines.
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