I did some experimenting with On Target. For my setup, I varied arrow weight in 25 grain increments from 350 grains all the way up to 550 grains.For every 25 grains of arrow weight,the modelshowed a speed loss5.135 fps (for my setup). This shows that On Target uses a linear relationship for speed vs arrow weight.

Next, I plotted my known chronographed velocities for various arrow weights ranging from 341 grains to 515 grains. When you compare the two graphs, you see that my actual chronographed speeds also follow a linear relationship. However, the SLOPE is different. In reality, for every 25 grains of arrow weight, MY bow loses 7 fps.

That's the problem in a nutshell. The software assumes a speed loss of 5 fps for every 25 grains of arrow weight. My bow loses 7 fps for every 25 grains of arrow weight.

I don't know why my bow would perform this way. The cam timing is dead on, and the measure DL and DW are correct. For some reason, my bow just "likes" lighter arrows.