I use a Camelback (70 poz.) plus one bottle (28 oz.) on mountainbike rides but on the road, just two 28 oz. bottles work for me, even on double centuries. Of course, on doubles you can count on checkpoints every 30 miles or so to refill bottles so usually, no worries.

In extreme desert environments, however (White Mountain, Death Valley, etc.) I will use a Camelback on the road in addition to the two bottles. Not much humidity out there, of course, so the sweat evaporates before you an even see it. Scary dry!

Rick / OCRR