[hat_man]

I'm not very good at using the Quote function, so this is a cut,copy,paste from the thread I linked above.

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.

If anyone has read the book cited in that thread I have a question......

It gives a range of of 16.4-19.8 ft/sec for best FE. Is this linear, with 19.8 ft/sec being "best of the best" and 16.4 ft/sec being "lowest of the best".....OR......is it more like a Bell Curve, with the middle (18.1 ft/sec) being best and tapering down both sides to both 16.4 and 19.8 being "lowest of the best" ?

Also, I'm assuming that the formula is 2 x ( stroke in inches) x RPM and then that whole quantity being divided by 720 rather than 2 x (stroke in inches) x (RPM/720).