Heck, I have a hard time keeping it UNDER 160 on a bike. Flat ground or no, unless I'm really loafing along, I'll average about 160, max out in the low 180's. I'm 37, so the typical 220-age puts my theoretical max HR right at the same spot.

I have read that the optical sensors in stuff like the Fitbit isn't always the most accurate for exercise monitoring, so maybe switch to a chest strap?