MPH speed from RPMs

[65Bowrider]

I found this bit of information on fiberglassics.com, and it really does seem to work! A good way to tell if you're going as fast as you SHOULD be going!

QUOTE:
Hector Gutierrez wrote on Wed Aug 1, 2001 3:39:19 PST in response to a posting on props:

Here is a handy little formula to calculate your theoretical speed for a given RPM. Us Mechanical Engineers have nothing better to do than to come up with this stuff. How close is your actual speed to the calculated? For my boat it was off by 1 MPH!

(Prop Pitch x RPM)
------------------- = Theoretical Speed in MPH
(Gear Ratio x 1056)

Example:

Prop size = 15" x 17"
Gear Ratio 1.68
WOT = 4,000 RPM


(17 x 4,000)
------------- = 38.33 MPH
(1.68 x 1056)

My actual was about 37 MPH