42/17 is enough for 20km/h surely
42x17 equals 28kph at 90rpm and 31kph at 100rpm, seems perfect for touring (if not a tad fast even perhaps).
http://www.bikecalc.com/speed_at_cadence
I don't find an average of 90-100rpm efficient for 7-10 hours of riding - the same point of view will you find in the audax thread.
@Thuekr started
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42x17 equals 28kph at 90rpm and 31kph at 100rpm, seems perfect for touring (if not a tad fast even perhaps).
http://www.bikecalc.com/speed_at_cadence