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For mowers with blades directly connected to the engine crankshaft, here's how to figure the feet-per-minute / blade tip speed:

L = blade length in inches
R = engine speed in RPM

L x R / 3.82

So, a 21" blade, with an engine spinning at 3,100 rpm means:

21 x 3100 / 3.82
65,100 / 3.82
17,042 feet per minute

FYI, most mower companies stay below the ANSI maximum standard, which is 19,000 feet per minute.
 

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Great reference!
 
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