Hi @teddygram! I’m going to share my data but you should be advised that the frame makes a difference and there is no other gravel bike that has a frame like mine. So, really, you should get out and test this yourself.
It’s always more aero to have the pack on my bike. Water bottles on the frame are really slow for some reason.
A couple of warnings. First, what you do with the drinking tube is very important. If you have a long drinking tube with a typical end valve & you leave it exposed to the wind (either just dangling down or looped out into exposed wind) then you are probably just as well off using water bottles on the frame. Keep the drinking tube close to your body…move it more to the center line of your chest and higher on your chest to reduce it’s impact on your aerodynamics…that’s what I’ve found. Second, what you do with straps is important. Don’t leave them flapping in the wind. Tape them up. If you can hide that behind your armpits, even better.
All my data was taken using a camelbak hydrobak. So keep that in mind.