I am yet to see any evidence to support this. On the contrary, there are plenty of articles that show the calculation is surprisingly accurate compared to real life (or at least, industry standard calculators such as BestBikeSplit).
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New individual outdoor 24-hour record is not that dissimilar to the current Zwift record.
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I think the issue is not the accuracy of solo ride speed. I find the issue to be the blob speed in Zwift. Somehow the riders at the front of the pack still receives a draft benefit in Zwift, leading to ridiculous speed differences between solo/small pack riders and a bigger blob. As a result, even if you output the same power as the guy who is pulling the pack, the pack ends up going faster than you do in Zwift. IRL, it takes coordination in the pack to catch up a breakaway, take turns and what not. None of this needs to happen in Zwift to catch a breakaway because of the “blob effect”.
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OK, I will match your hundreds of races (at 1059 today).
Breakaways are reasonably common. Although it can depend on the race setup. For example my last two races were small 3R Classique x 5 laps. Both races I started in the A group with about a dozen other people, similar numbers in B and C. Within the first two laps in both events the race evolved into a break of 3-4 riders (mixed A/B), first chase about 8 (mixed A/B), then everyone else (small groups of C and D).
Both of the WTRL autocat test events, large (40ish) starts of mostly high B, some low A (C3) and both races the field split into the breakaway group (dozen plus) and the rest in the first or second lap.
I have seen numerous races where a strong rider will make an early jump at roughly 800-1000m to go, to get out front and win (if they don’t get caught.)
In Zwift the Blob (usually) wins. But not always.
You mean…like in real life? 
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Zwift’s physics, other than drafting which has some fundamental issues (pack position determined by client rather than server), are fairly good. In real life, W/kg is what determines speed on climbs, while W/CdA is more important on flats, where CdA is proportional to cross-sectional area, and cross-sectional area has a component from the bike (bike size depends on rider height) and the rider (where there’s height, but also a contribution from weight since width depends on weight). While it’s true people some people lie about their dimensions, it’s more realistic this way. And it’s also true some people obsess about weight, but it’s not really any different IRL, since real physics also depend on weight.
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Every season more shorter and lighter people!!!
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If we look at the classifications on Zwiftpower, we can see in top positions more than a half of people with a “difference of 20” or around it. That is:
190cm 68kg
183cm 65kg
180cm 62kg
175cm 56kg
And even extremes of males with kid dimensions (150cm 45kg…etc)
In real life, this will never happen, because kids are in clear disvantage on the flat and descents, they are no able even to keep in a peloton.
All of this is because the wrong conditions of Zwift physics.
But that’s what the GAME wants to meet.
Publicity on TV is: “fun is fast”, or about “flying in the climbs” that is actually possible for ALL the people INTO the GAME, and for less happiness… there is real world.
Zwift is business. A “dreams of flying with your bike” business.
A while back I did a set of rides with different weight/height/power to calculate the relationship to speed in zwift. The answer is quite accurately modelled by
P = 1.86e-02 w.v - 5.37e-04 v^3 + 2.23e-05 w.v^3 + 1.33e-05 h.v^3
where v is the velocity in kph, w is the rider weight in kg, and h is the rider height in cm.
This is all calculated for a flying solo draft-free lap of the volcano circuit, so the terrain isn’t precisely flat but it’s not too hilly. I did a few checks on properly flat terrain and in that case the equilibrium speed is about 0.5kph faster than this formula suggests but the nature of the relationship with height and weight is very similar.
The cubic terms in the model show that height and weight both affect frontal area, as well as weight impacting the (linear) rolling resistance term. I have no particular view on whether the impact of height and weight on frontal area is realistic or not, but many others suggest they are a bit too strong.
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It is quite simple - ask people what speed they tell people they can do in real life and then just halve it.
For example, on the flat I’m around 2km/h faster in real life at 300W than Zwift.
I was just joking and suggesting cyclists like to exaggerate 
Does it really matter?
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If everyone is affected by the discrepancies the same way, it does not matter. If Zwift’s algorithms favor one type of rider over another, it does matter.
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That’s it.
Shorter and lighter are faster in Zwift than real life.
Taller and heavier are slower in Zwift than real life.
That’s why you see “diference of 20” (185cm 67kg, 175cm 58kg, 170 cm 52kg…etc or even many “kids” 150cm 45kg) in a half of riders racing at Zwift.
Strange population!
Dream game for (tricky) wannabe fast riders, but quite annoying for real people.
Changing your personal data to be shorter and lighter will inevitably make you faster in any reasonably credible cycling simulator, regardless of how accurately their calculation maps onto real world behaviour. It’s certainly appropriate to use both height and weight for calculation of aerodynamic resistance (since they relate to rider size/shape) and also weight directly influences rolling resistance.
Well, that’s true. Also in real world.
But in real world doesn’t happen that a kid 155cm 45kg is as fast as a male of 185cm 80kg in descents, because “the influence of weight and height in aerodynamics and rolling resistance” is quite different than Zwift 
In a Zwift ITT on flat terrain that kids also beat UCI pro tour riders… so it must be something wrong…maybe?
But Zwift is not that “credible cycling simulator”, because it’s a game.
Gamers usually play doing things that never can do in real life.
So, as TV spots say: “fun is fast”, and enjoy flying in the famous Alps climbs.
That’s all.
And hear we go again.
I do agree Zwift has to make some assumptions and kids seem to be faster than IRL but faster than IRL, it think this is why Zwift had the weight limited for such long time. Dropping the weight limit seemed trivial but the consequence was that the math does not translate well for low weight.
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I only was using kids for dramatize, we can forget that extreme and talk about “difference of 20” riders and the short&light massive population of Zwift riders taking advantages.