Bottom line is you have to figure out what works for you. Everyone deals with heat differently so it is pretty silly to try to come up with a general rule. I am a very heavy sweater and in the summer when the RH is 80% I can lose 5lbs on a 1.5hr ride that means 2 bottles for anything over an hour ride. In the winter it is not even close when the RH is <60% I will lose <2lbs so 1 bottle is sufficient. That is what works for me. I have come up short a couple of times and believe me I much rather carry extra water than deal with dehydration.
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The problem with the gene pool is that there is no lifeguard and the shallow end is much too large
2013 Noah RS