Hi
Ghurd is correct. The constant for reading RPM on the Kph scale is 166.7cm (or 1667mm). Remember, the diameter of the prop is totally irrelevant. All you want to know is how many times it is turning per minute. (Now, if you wanted tip speed, well that's a whole 'nother kettle of fish.)
Using the Kph scale is preferable to using the Mph scale because a speedometer running to max 100 kph may only be capable of showing around 60 Mph max.
Owen
If you're not living on the edge, you're taking up too much space.
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