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I love my Zip, and one of the reasons is that when I'm cycling, I either put it in the leg pocket of my shorts, or tuck it into my sock, that way it counts each pedal revolution as a step.
Whilst not perfect, it will register a lot more steps than if I had it in a waist pocket, or if I had a wrist worn Fitbit. For your 18 miles, I would probably have gotten about 8000 - 10000 steps, which I feel is a fairer representation
Fitbit Product Experts Alumni are retired members of the Fitbit Product Expert Program. Learn more
A lot of people find they get more steps when the fitbit is attached to the ankle in some way. Just a note of caution though, it is likely that the calories burned will be overestimated. Not a problem for many people but for those using fitbit to manage weight it may be an issue.