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Here you go;
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.
OR:
Piston speed = 2 x Stroke in inches x rpm / 720
Please keep in mind this is only a rule of thumb and specific situations will alter the "rule".
That said it is a good starting point.
The source by the way is Taylor as linked above.
A good book too since it has all the necessary info in one place and is more up to date than many others including Judd and the now classic Ricardo publications.
Cheers , Pete.
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