Generally the IBO speeds will fluctuate a few FPS different. Dropping to a 60 pound bow, it will usually be a few FPS slower on average, 50 pounds a few FPS slower than that and on an 80 pound a few FPS higher than the 70 pound rating, though that' s not always the case.

I have an 80 pound Liberty ordered (hopefully getting it very soon), but I' ll be shooting @ 29" . As soon as I can chrono it, I can let you know and then you can extrapolate an appx. 80/30 speed.