I've been told that whatever max HR you see while out on your bike, your real max HR will be a few percent higher.
The theory, which has apparently been demonstrated, is that you are always going to be able to stretch your effort more under lab conditions, i.e. on a bike in a lab with someone coaching you to your effort, than while out on the road, where other variables, like distractions of environment, will inhibit that.
Consequently, you will be able to squeeze out a few more BPMs than you might expect.
The most I've seen recently, over a period of six months, is 186 (paltry considering I used to max out at 210 when I was in my 20s!) and the suggestion is that my max heart rate is more likely to be around 192.
What do other people 'know' based on their learning and wisdom; is this theory correct?