Accuracy of max HR

Your real max heartrate could only be found out with a test (cardiogramm) at doctor.

I've been training with a chest strap for almost 20 years, at first using Polar watches and since 2015 Garmin. Each of them has given me high readings from time to time but luckily it is very rare. The two most common reasons has been static electricity (between clothes and the HR sensor) or almost empty battery. I very rare cases I've got high values while running or cycling along a power grid. My tip is to change the battery and to make sure that your clothes are damp around the sensor (to avoid static electricity).

When I look at your activity I would say it looks normal to me. When I get high values it is constant, for example 190 for a couple of minutes and then back to normal. But your graph looks smooth and constantly changes. Perhaps you just had one of those days when you were able to perform at a high level.

My advice is to have it calculate your lactate threshold and use %LTHR for your zones.  I've found it to be more accurate, considering that my max hr is on the higher end.  Other ways ot estimate your max hr can be found online and involve sprinting uphill for 3 minutes at least 3 times (not rest) until your hr stops increasing at the end of each bout.

That 220-age formula is really conservative - many people have much higher max heart rates than it suggests. I’m practically old enough to be your dad and mine is 198. The graph looks reasonable. Might be that your MHR is a bit higher than 195, it is very hard to maintain 95% of your MHR for long. 

As mcbadger says but to go a bit further. The 220-age calculation is just one of many methods to estimate your max heart rate. It tends to under read if your young and over read if you are older. There are many other formulae out there some of which might give you a better estimate than others. But that's all they are, an estimate. If I use 220-age my HRmax comes out 156 when in reality it's 30 beats higher than that.

Best thing to do is to get a HRmax test done in a lab using 'properly' calibrated equipment. Next best thing to do is to undertake your own HRmax test - www.runnersworld.com/.../

Note that in general, the HRmax reading is not usually an instant spike. In labs we typically take the highest 5s average recorded.

Garmin uses the 220 - age formula by default.

If you know your true max heart rate, just change it on the watch and your zones will adjust to that.

According to the 220 - age formula my MHR should be 158 (I'm 62 years old), but I have gotten my heart rate up to 170 multiple times so that is what i have mine set to.

Garmin uses the 220 - age formula by default.

I think the watch automatically records max HR and LTHR based on activities, and saves this data in the user profile. So the highest I have got my HR up to with the Fenix 6 is 194. But I have had it up to 200 in the last 12 months with my previous 645.

What confuses me though is why Garmin doesn't have a race mode as I always get "Overreaching" training effect from going at race pace with a high average HR. Obviously you are not running to heart rate in the later stages of a race, and some people like me just have a high suffer threshold.

You should probably set it to 200 then.