It depends on a lot of things - what your bike is and what your wheels are for starters (tube shapes, bottle boss placement, etc). There have been cases where it actually shows as faster.

'13 shiv s-works w/808s

I recall that on the cervelo tests that even the most aero bottle still caused a little drag. The only bottle that they tested that showed reduced drag was P4 with P4 bottle.

Their conlcusion I think was bottle between arms and bottle in correct position behind you was best.

For me, I still go bottle between arms and skinny bottle on seat tube (in practice I find it takes me too long messing around with the behind me stuff, so for me it is probably slower).

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