Any way of tuning race predictor?

It is predicted based on your vo2max and training, also I would say that 'impossible' is not the right word for any sport.  

Even once I started training pretty 'good' for me for 5k's back in 2014... i was running them in 22min, 7:11/mi... well just a handful of years later with jumping to Marathon training and adding in biking (30-150milers)... 6:58/mi is my Marathon pace!!!  structured and focused training can change much!  

Not sure how much training you are doing now and how structured it is, how much of an aerobic base you have... but there is obviously more you (anyone really!) can do in just 5-8weeks of buildup to a race.  Also a Time Trial solo or even with a small group of friends is much much different then an actual race!!!  especially a big race with family/friends watching cheering!!

I'm running about 80km/week. It's not very structured, with just a 5k TT, 10k TT then easy and long runs typically. I understand I'd get faster if I follow more structured training, but then it will bump up my vo2max and predict an even faster time. Anyway, garmin knows everything there is to know about my training, weight, body fat%, age, max HR, LT and cross-training. I don't see why it's so inaccurate for me.

Ok, so you are comparing your weekly training workouts to your race predictor?  That's apples and oranges!    There is no way when I ran my PR 5k that I could have done that without it being a race, having people cheering, competitors behind me and in front!

Sign up for a 5k race... train a few weeks of more structured workouts for a 5k (with some hill sprints, and 3-5min intervals harder than 5km pace but easier than 1mile race pace with equal jog recoveries, and ~300m Speed Reps)... shorter long runs, taper the week before race.  Then see what is the what.

BUT - yes, Garmin overestimates pretty much everyone's predictions based on their vo2max that I have seen.  My 5k PR is 18:22, predicts 16:44 and I am running less now and limited to none speed work.

No, I'm comparing the race predictor to what is possible for me in my current state of fitness and training. Everything that Garmin knows. If I do optimal training for a 5k, obviously I'm going to be faster, but equally obviously, Garmin is going to predict even faster. All I'm asking for is a way to detune the prediction so that it is actually meaningful for me, just like I can do with runalyze.

BTW, the 5k TT is, for me, a race in everything but name. I don't put much stall on being any faster with other competitors, I'm only competing against myself in whatever type of event it is.

If you have been doing 80km a week including 5km paced workouts and 10km paced workouts most weeks... most likely if you reduce you volume and add in some speed work above 5km pace Garmin won't think much different and in a few weeks your vo2max won't change much if at all.  You economy/velocity@vo2max/legstrength will likely improve with change of training stress.   Also the 'predictor' adapting to your recent training... i haven't noticed much in that.... mine stays stuck at the same with varied workouts.  I get it, I have done TT and/or can push workout days super hard as well, but with a taper on fresh legs... can always find another gear come race day!  Heck with training fatigue I'd be lucky to hold my 5km pace for 2miles probably.   

I am assuming on some of these TT you are doing some bit of taper in the week/weeks leading up to them to get a actual rested recovered race effort in?   Otherwise then it isn't really even a TT, let alone a race.   

Also it isn't that much about faster with competitors, its also just race day energy, excitement, butterflies, leadup to the big day you've been preparing for weeks/months for... some people don't like crowds or manage stress and can do worse in Race.

Just to reiterate. My question is not how do I reach the race prediction time. It's how do I increase the race prediction time to what I can achieve right now? I don't care about the training to optimise my 5k race time. I want to be able to increase the time it is predicting. i.e. by reducing my supposed vo2max to something realistic. As is often said, the garmin vo2max is typically 3 points above your real vo2max. I tried setting my vo2max to 47 from 50 in the web config, but it doesn't seem to do anything.

Thats weird you say +3 vo2 readings... I think everytime I have seen a mention of someone doing a lab test it was within 1pt.  You have tested yours recently against the 945 #'s?

The predictor is such a minor component of what the vo2max, training status, training effect Aero/Anna and recovery are about... so to falsify your maximum heart rate down or weight up to get your vo2max to drop to get your 5k predicted to match you workout 5k pace... doesn't make much sense.  To try to fit your desire for what that number is... vs what they are using/telling you is just going to mess up things elsewhere.  

If you training is so consistent and steady... race times are not going to change much.  use daniels vdot calculator or mcmillians if you want to know what your predicted race paces are at other distances based on your time trials.

But, to answer your specific issue... raise your weight until your vo2max is what you want it to be...  or if track your weight w/garmin.... instead lower maximum HR until you get the answer you want.  Adjust HR zones to match reality (since max HR is fake... will want to adjust HR zones to what they would be with real HR max).  or... just use Daniels VDOT website , probably easier for the once every couple months fitness change.

No, I haven't done a vo2max test. Runalyze gives me an effective vo2max of 42.5. An online calculator gives me 42 from my 5k time. Garmin gives me 50. Unless I'm strolling around that track in zone 1, 50 is a fantasy. My weight goes from withings to garmin connect to runalyze and other places. I'd rather not have to disable it.

If you want it accurate stop doing cross training. It makes you fitter but not faster. Your engine is too big for the chassis. Your economy and leg strength are your weakest link and that bottlenecks your 5K

What could be happening is incorrect Max HR.  The watch “thinks” you’ve got more in the tank. What’s your max HR on the settings V the TT? 

Remember the prediction is just that. Perfect weather, pacing etc. Once you tinker with the HR max you should see more accurate times. 80km is solid. How often do you run that? Big difference if it’s the average for last fortnight or for the last 10 plus weeks.