What is more aero? one or two bottles?

One big (750ml) or two regular (550ml)?
(in my case Venge)

You may find https://www.aero-coach.co.uk/water-bottle-testing interesting.

Short version, round bottles of any size on the down tube are poor aerodynamically. 1 large bottle on the seat tube is the best option.


Nowadays it is very much frame dependent, as bottles are taken into account with the aerodynamic design.
On a tradition round tubed bike, it is true to say that a single large bottle on the seat tube is more aerodynamic than two smaller bottles, or a single large bottle on the down tube. Most of the newer aero bikes though have been designed to provide a degree of shielding to a bottle placed on the down tube. For example, the current cervelo s5 and Pinarello f12 are both designed with bottles in mind. On bikes like these, a bottle causes less aerodynamic drag when on the down tube. Most of this design though is based around a single small bottle, so whether a single large bottle on the downtime would be better than two small bottles is more of a question to be answered.
The aerocoach testing is interesting, but they have used a frame that is not really designed to carry any water bottles in the main triangle. It may be the case that a frame that is aero optimised to such an extreme scenario is more negatively impacted by a bottle than a standard road bike would be. Basically, their testing is great but it is completely unrepresentative of the vast majority of road bikes. It would be great to see them replicate the testing with a standard, non aero optimised, bike

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Specialized claims that if you want to use one bottle then use it on down tube (although at the beginning they said that is better to use seat tube, but after they said that was a typo)

Depends on the bike.

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When I tested a round 710ml bottle was faster on the down tube. According to the guy at the tunnel that wasn’t uncommon, at least on modern TT bikes with truncated airfoils.

Far from perfect advice, but in general if your bike has a downtube with a flat back (truncated airfoil) then it’s probably more aero with the bottle on the downtube, as long as the bottle is narrower width than the downtube.

Otherwise put the bottle on the seat tube or mount it under the seat pointing backwards. However, bottles mounted under the seat almost always fall out unless strapped in (ask me how I know).