if i changed from my 85gr broadhead to a 100 how many fps do you think i will be losing.

not much i normally use 100 grain heads and i think for most thats as heavy as you need for deer / elk plus there are a lot more heads offered in 100 grain models than in 85 grain heads

15g will be almost unnoticable as far as speed goes. I think it would be less than 5fps. Just curious, Why the change? Are you trying to obtain a better foc or just something new?

no real reason for the change i just happen to have 6 brand new 100gr thunderheads that i shot last year out of my old bow and when i bought a new bow i switched to the 85 gr to try to get a little more speed.

5-7 fps

It definitely won't be much. I'd say on average about 4 FPS.

You shouldn't see more than a 5 fps variation.

around 5 fps