The doctor friend I ride with tells me that 220 - age is completely bogus. Also, there are a large number of publications that also say this. That's good, because my max heart rate (measured through an escalating effort time trial) is at least 167, whereas 220 - age tells me it should be 155.
Here's one of many papers on this:
Despite the acceptance of this formula, research spanning more than two decades reveals the large error inherent in the estimation of HRmax (Sxy=7-11 b/min). Ironically, inquiry into the history of this formula reveals that it was not developed from original research, but resulted from observation based on data from approximately 11 references consisting of published research or unpublished scientific compilations. Consequently, the formula HRmax=220-age
has no scientific merit for use in exercise physiology and related fields.
https://eprints.qut.edu.au/96880/
Supposedly, a better max HR formula is 208-(0.7 x age), but this is still way off for many fit people. This tells me that my max HR should be 163, but I can consistently get to 167, so it is at least 4 beats off.