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All the AMO speed ratings are given for 30 inches of draw length. I have only a 28 inch draw. How much speed is lost for each inch of draw length? Does percent letoff change for short draw lengths?
For every inch either side of the AMO standard that you draw your bow you affect your speed by approximately 4 to 5 %. To calculate how fast a bow will shoot at your 28 inch draw with a 540 grain arrow and 60 pounds draw force, multiply the bow's AMO rating by .92 and .90. This is your bow's base speed range. For lighter arrows add one fps for each five grains of weight you remove. For heavier or lighter draw weights, add or subtract 2 fps for each pound of draw force change. Letoff should not be affected by draw length except at very long draws with only certain cam designs - in this case it will be less.
Original Article: Whats the Relationship with Arrow Speed and Draw Length?