Assuming a 205mph ball speed and perfecto contact; you are looking at 136.6mph of swing speed. However this isn't realistic; more than likely the contact is more at the 1.475 rangish which is a figure i have come up after lots of research to be a more realistic number of efficienct contact and you get a swing speed of 138.9mph.
But that realistic number is based more on shorter 45" drivers and very good players who aren't swinging completely "all out." So i would tend to lower that smash factor for a long driver to maybe 1.45. Using that you end up with 141.3mph of clubhead speed for a 205mph ballspeed of a long driver using a much longer shaft and also the effect of going "all out" on every swing.
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