When you compare your Zwift speeds to outdoor speeds, which is faster?
Zwifters tend to find their speed in game is faster than their speed outdoors, typically by a margin of 1-4km/hr. This can be disconcerting since many riders set fitness goals based on miles ridden per week.
There are several possible reasons why your outdoor speeds could be slower than your Zwift speeds:
- Zwift’s algorithms for calculating speed are incorrect.
- Zwift’s calculations include assumptions (such as your position on the handlebars) which do not reflect your outdoor rides.
- Outdoor rides are slowed in ways Zwift rides are not.
- Zwift gives you more opportunities to draft.
- Your power readings in Zwift are not accurate, leading to unrealistically high speeds. (This topic needs a separate article by itself, so I won’t deal with it here.)
Let’s look at these possibilities in detail and figure out what’s really happening.
Possibility #1: Zwift’s Algorithms for Calculating Speed are Incorrect
Formulas for calculating bike speed are fairly established, straightforward and reliable (see some details about that here). I highly doubt ZwiftHQ’s experienced game physics programmers have made any major errors in this area, so I’m going to dismiss this as an issue.
Possibility #2: Zwift’s Calculations Include Assumptions Which Do Not Reflect Your Outdoor Rides
Do you spend most of your time on the flats, on the hoods, or in the drops? How high is your seat, and how tucked in are your elbows, knees, and head? How smooth is the pavement?
There are many factors which effect your bike speed apart from the basics of road grade, power, weight, and CdA. Zwift has to make some assumptions here to keep things simple, and based on what I’ve seen, those assumptions are a bit generous.
Using our test lab data, I looked at some basic numbers for the 7.6km flat section of the Richmond course (see Strava segment). (No, I didn’t run the numbers for climbs, or group rides–I wanted to keep it simple.) I used two different online bike calculators, setting them to 25 degrees Celsius, 50m elevation, 8kg bike weight, and default values otherwise. Here are the numbers:
- 300 watts, 100kg rider weight
Zwift result using Zwift Carbon, 32mm carbon wheels: 37.9km/hr
BikeCalculator.com: 36.17 on hoods, 39.16 in drops
Kreuzotter.de result: 37km/hr in drops - 225 watts, 75kg rider weight
Zwift result using Zwift Carbon, 32mm carbon wheels: 36km/hr
Zwift result using Zwift Carbon, 32mm classic wheels: 35.9km/hr
Zwift result using Cervelo S5, Zipp 808 wheels: 36.7km/hr
BikeCalculator.com: 33.03 on hoods, 35.77 in drops
Kreuzotter.de result: 34.7km/hr in drops
- 150 watts, 75kg rider weight
Zwift result using Zwift Carbon, 32mm carbon wheels: 30.7km/hr
BikeCalculator.com: 28.26 on hoods, 30.55 in drops
Kreuzotter.de result: 29.8km/hr in drops - 150 watts, 50kg rider weight
Zwift result using Zwift Carbon, 32mm carbon wheels: 33.4km/hr
BikeCalculator.com: 29.01 on hoods, 31.44 in drops
Kreuzotter.de result: 31.5km/hr in drops
As you can see, Zwift speeds are consistently 1-2km/hr higher on average in the lists above. But here’s the interesting thing: small changes in the bike calculators (such as using tubulars instead of clinchers, or a slightly improved CdA, or a higher elevation or temperature) are enough to equalize the speeds.
Additionally, the bike calculators don’t let you set which bike frame or wheelset you’re using, and we know those can make a significant speed difference as well.
Overall, I think it’s safe to say Zwift makes some slightly generous assumptions about your posture, equipment, and environment which do not perfectly reflect a typical outdoor ride experience. The difference these assumptions make in speed is reflected in the numbers above.
Possibility #3: Outdoor Rides are Slowed in Ways Zwift Rides are Not
Especially for riders living in high-population areas, ride speeds are impacted by traffic stops and slowdowns. Stop signs, waiting for cars, braking on steep descents, slowing to turn or avoid obstacles–all of these are common occurrences outdoors which never happen in Zwift.
Then there’s wind, which doesn’t exist in Zwift but plays a major factor outdoors, rarely helping but often hindering our speed.
Consequently, your Zwift average speeds will be inherently higher than your outdoor speeds.
Possibility #4: Zwift Gives You More Opportunities to Draft
Drafting in Zwift, like drafting outdoors, results in a power savings of ~25%. If you are comparing your outdoor solo ride speeds with Zwift speeds, the draft effect alone will make a noticeable difference, since you often draft in Zwift even if you’re not taking part in a group ride.
Conclusion
While a Zwift ride will generally be faster than an outdoor ride of similar effort, the differences are minimal and not enough to invalidate including Zwift miles in your training metrics.
The real question is: could I get the green jersey outdoors? (Probably not.)
The reason why I don’t wear a helmet on Zwift island is the same reason why I believe Zwift does not need to reflect the real world 1:1.
What counts are watts, and they are quite comparable to what I push IRL.
Everything else are just that extra KM/miles on my strava profile 😀
I agree – Zwift doesn’t seem overly ‘fast’ compared to my IRL riding – Watts vs Gradient vs Speed seems to fall into line fairly well – given that Zwift does not have wind affecting the output speed. I do believe, however, that Zwift draft-effect, particularly in BIG bunches, is somewhat better than IRL – though how much, i cannot say. Great article!
Not to mention the possibly even higher speeds when in a large group (aka blob!)
Does Zwift pay attention to wind, road surface and rain/snow? If it doesn’t its like riding in vacuum. That’s would explain greater speed..
Regarding “POSSIBILITY #3: OUTDOOR RIDES ARE SLOWED IN WAYS ZWIFT RIDES ARE NOT”, I mostly look at my 75% percentile speeds IRL. Rhat way you rule out the trafficlight stops and GPS errors. And to my experience my personal “fatigue” state them matches the figures 😉
Yes. Even after taking into account all mentioned conditions like constant drafting and non existent braking, Zwift speeds are unrealistically fast. I think Zwift assumes unrealistically low CdA, Crr and Rho parameters.
Road surface smoothness certainly has an effect. Streets on Zwift are perfect. I haven’t noticed a difference between riding on Zwift’s gravel roads and riding on Zwift’s asphalt roads. And not only that: we will rarely pick the optimal line.
25 degrees is a very optimistic pick. I think the majority of us will encounter these temperatures in the minority of their days. Even though higher temperatures (as recently encountered at the Tour Down Under) will decrease power output for other reasons, and hence would slow us down again.
I also think we tend to underestimate the actual riding weight of our bike. To the weight of yourself and your bike, add clothing, food, pump, tools, water bottle(s), electronic equipment, keys… I don’t know if Zwift incorporates any of these, but somehow I doubt. I expect Zwift’s weight to be the net bike weight plus the rider’s input weight. The difference between the system weight in Zwift and outdoors therefore will easily reach 2kg. This one doesn’t show in Kreuzotter’s calculation, but consider that you need to accelerate the extra weight after every turn.
Finally, my outdoor rides tend to be longer than my Zwift sessions, and the longer the ride, the less I’m inclined to go full gas early on. The longer the Zwift session, the less I’m capable of maintaining the same power level.
One other factor is coasting. I rarely, if ever, coast while riding indoors. Outside, unless I’m in a race or fast group ride, I often coast or push many less watts when descending. I imagine that is also a factor. So, as others said, I rely on watts.
I analyzed my last 200 rides and found Zwift to be exactly 1.0mph faster than my outdoor rides. I’m chalking that up primarily to #3. I never have to slow down for anything on Zwift.
Wind is also a non-factor because once you’ve traveled 360 degrees it cancels out. The only thing that’s not realistic is that on Zwift you never get those awful terrible days where the wind shifts half way through your ride so you do the whole thing against the wind. In most cases, though, it’s fair how it is currently done.
i agree with this, i seem to be about 1 MPH faster on Zwift, maybe 1.5 MPH
There’s also the difference between riding a Tron bike in Zwift and a round-tube brick in the real world. My son & I were doing “recovery” rides at 20+mph on Zwift. I had him switch to the steel bike on classic wheels, and it made a HUGE difference that much more accurately reflects his real-world abilities.
I’m still faster outside. 100mile ride i averaged last year 25mph with a group. On zwift even with a lot of drafting 23.
Solo I get about 22mph up to 2.5 hours. On zwift I average 20-21 solo. A little faster working with a small group and I use the tron bike on zwift. Before the Tron bike I was about 1.5-2.5 mph slower on zwift.
I use a powertap hub for both real world and zwift. I understand smart trainers seemed to be programmed with a extra watts. Also single leg powermeter can be way over or under on watts based on your dominant leg. That being said, below 2 watts a kilo on flat appears to be faster on zwift and downhill appears to be slower also. I mean when can you coast to a stop on a downgrade of 2-3%. I usually accelerate on a coast to. Uphill seems to be slower than real life. So they tend to balance out if you do a little climbing.
Also my sprints are way way more in real life by upward of 300w.
Zwift probably gives slower heavy riders an advantage and slimmer more powerful riders a challenge, to keep it more interesting for everyone.
I’m pretty sure that your guess is correct 🙂
“Zwift probably gives slower heavy riders an advantage and slimmer more powerful riders a challenge, to keep it more interesting for everyone.”