No repeal of physics necessary, Bob; only a chronograph and actual shooting of bows is required. Since you're going to follow me around, you might as well learn something.
I've a couple compounds that, in their day, were pretty danged fast (20 years ago). Set at 80#, a 500 grain hunting arrow gets shot just as fast as the Prime I'm shooting now set at 60# (that's the bottom end of a 60-70# Prime, for reference as well).
The wife of the local pro shop shoots an Obsession (he's an Obsession dealer) set at 53#. It shoots a 450 grain hunting arrow for her just as fast as either of the older bows I have set at 70#. In fact, given how much draw length she is giving up, if that were equalized, her bow would actually be faster.
Those situations aren't unique today; in fact, they are very common.
BTW - I did NOT say a 40# bow of today equals a 70# of twenty years ago. I said a modern compound of 50#, or even 40#, would give you what a 60-70# bow twenty years ago could. 40# bows today can, and often do, shoot heavy hunting arrows (400+ grains) at speeds equivalent to what a 60# bow twenty years ago was delivering with the same weight arrows; and 50#s now can do what 70# did then as well.
IBO speeds from the mid-1990s (20 years ago) were, at best, 300 fps. In fact, a lot of bows were still being rated AMO (60#, 30", 540 grain arrow), and those speeds were in the 220 fps range. The reason for overdraws back then was to drop the arrow weight without reducing the spine to achieve higher speeds. I know; I was hunting and shooting bows back then (and before then).
Today, 340 IBO is pretty easy to achieve. Go see for yourself how fast a mid-1990s 70# compound shoots a 500 grain arrow, and then shoot a 500 grain arrow out of a current compound at 50#. You're going to have speeds in the 230s. Shoot a 400 grain arrow out of the same bows, and you're in the 260s.
The arrow, nor the chronograph, care what poundage you're pulling, only the speed at which the arrow is launched. No changing any laws of physics, just tracking efficiency.
