Someone will say i am only pulling 45#'s but with how fast todays bows are I am not having any penetration problems. Does that mean 45# 20 years ago is not the same as 45# today??

In some cases yes and some No. It depends on the bow you arte shooting. I know of some older bows that perform as well as todays high tech and special limb material bows. If you are shooting whitetailed size game 45#s will kill a deer just as dead 20 years ago as today. No Doubt! Shawn

Limb efficiency is what makes the difference. A more efficient limb will shoot the same (or even heavier) mass weight arrow faster at the same draw weight as a less efficient limb.

What Shawn said   .

again...what shawn said.  you can't kill em eny deader than dead.  

however the real answer is to do a force draw curve of your bow, and compare it to the f-d curve of older bows to see the stored energy.