Having said that, again only a rough guide at 50 my max HR is supposed to be around the 170bpm, but I regularly hit the mid 170's and a bit higher sometimes, and never felt its the limit.
I agree, my max HR is supposed to be 176, although it's only based on the formula 220 - age, I can hit this no problem on the steep hills and hit into the low mid 180's for short bursts before my lungs explode although I cant feel my heartbeat, I can ride in zone 2 depending on the ride, althougha bit more in zone 3, To be honest I ride fairly steady av.sp.12.5 mph over 50 miles, with probably about 5-10% in zone 4 and occasionally 1 or 2% in Z5 when pushing hard up steep hills
If I'm on the Rail Trails (mtb) with my dog running along side I can be in zone 1 (<105bpm) quite a bit and Zone 2 and not hit zone 3 at all at av.sp 8.5mph
I did speak with my doc regarding the high HR, the fitter you are, your HR max will increase and also the fitter you are you can sustain lower HR's in exercise, however this depends on the exercise, if you ride killer hills busting your lungs every time all the time your never going to be in low zones, Personally I ride steady to achieve bigger miles rather than short miles with killer workouts.