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.)