Vo2 Max / Race prediction times way off???

This is something that has been discussed at length in different parts of the forum and if you do a search of the whole forum with your thread title you'll find plenty to read on the topic, but the gist of most of them boils down to:
A: You need to make sure your HRMax is accurate for you
B: Most people find that if they subtract 2 from the Vo2Max on their watch and look up race times in a chart (Coopers maybe?) it will be closer than what the watch says.

The key of course is having your HRMax set correctly or everything is out the window.

The key for an accurate prediction is HRMax and it should not be set automatically as it varies a lot from one person to another. Your HRMax for running should be the "realistic", i.e. the HRMax you hit at the end of your 5k for instance. Let us know what the numbers are.

During my last 5k, my average HR was 169 and my Maximum was 178 (being 45 my textbook HRMax is 175 so about right).
My average resting HR for the last 7 days is 52. I assume you mean I should enter these rather than use the auto setting? Do I switch auto Max HR detection off ? (why have an auto setting?)
I read some other threads, but no ones seemed as far out as mine which is about 5 to 6 out if you match to my PR's.

Yes 178 would be a good number to start with in your Running HRMax. What's strange is that the auto should be 220-45 (your age)=175 so lower than 178. On the other hand if you're using the OHR it might have been bumped up to 200 or something...

Just changed it to 178, it was 175 - was this set by the auto detection? The Vo2 max didn't change.

It was set using your age, see my previous post.
Yes the VO2Max gets calculated each time you complete a run and is smoothed so takes a few runs to settle. But since you've input a higher value it should go "up". Now was that 178 correct ? Is that from a strap or from the OHR ?

This is something that has been discussed at length in different parts of the forum and if you do a search of the whole forum with your thread title you'll find plenty to read on the topic, but the gist of most of them boils down to:
A: You need to make sure your HRMax is accurate for you
B: Most people find that if they subtract 2 from the Vo2Max on their watch and look up race times in a chart (Coopers maybe?) it will be closer than what the watch says.

The key of course is having your HRMax set correctly or everything is out the window.

I agree with B. I've found that my estimated Vo2 Max on my watch is too high by about 2...which equates to about 22 seconds on a 5K.

The 178 is from the built in HRM on the 935, I don't have a strap HRM, so appreciate it may be out, though the Vo2 Max hasn't changed since I altered Max HR from 175 to 178.
Sounds like all the settings are ok, how much of the 5k race should my heart be going full tilt? An average HR of 169 would indicate I'm trying pretty hard!

Just had a look at some older runs imported to garmin connect - the HR figures are a bit 'out there' - maxing out at 200 in some cases. This was from an in ear headphone HR monitor which was a bit dubious. Would these older runs have an effect on the current Vo2 max?

Yes it's normal for the VO2Max not to change, see my previous post on how it changes ;-)
169 seems plausible for the average HR of a 5k, but you should look at the HR graph to see if it's consistent. I've permanently moved to the HRMRun4 as I was tired of the guesswork that comes with OHR (I gave it a long try though starting with the FR235 back in 12/2015!)

Not sure about that "2" substraction. The trick is really to use your "Current Fitness MaxHR" under Running. When I do that I get predictions that are close enough (except for marathon but that's the same with the Daniels tables). What's nice with the FR935 (unlike the FR235) is that you can keep the "Absolute MaxHR" value and then adjust the "Sport HR" for running or cycling. Unless you're a pro cyclist you're unlikely to hit the same MaxHR in cycling and running.

For a 5K I average about 91% of Max HR when going flat out. I can hit close to my Max HR in the sprint for the line. The 2 subtraction works for me. My current VO2Max is showing at 59, which equates to 17:22 5K. My best is 18:59 this year.