Are Zwift’s Tarmac Speeds Realistic?

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 for solo rides. 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 or the condition of the road) that 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 is a separate topic which we won’t deal with 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 well-established and reliable (see some details about that here). Zwift simply feeds a variety of variables in such as:

• Rider and bike weight
• Rider CdA
• Aero settings for equipment
• Tire rolling resistance
• Gradient
• Air density

into an established equation for speed, and the correct speed gets spit out.

I highly doubt ZwiftHQ’s 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 other factors that affect your speed apart from the basics of road grade, power, and rider weight and height. Zwift has to make some assumptions here, and it’s fair to say those assumptions are quite generous.

One example: all Zwift tarmac has the Crr of new tarmac. How often do we get to ride on new pavement outdoors? It rolls fast.

We ran some speed tests on the Fuego Flats forward sprint with different rider weight and watt variables. (No, we didn’t run the numbers for climbs or group rides–we wanted to keep it simple.) Rider height was set to 175cm, the average male height in the US. We used timings from a basic Zwift Carbon frame/32mm wheels setup as well as a very fast setup of the Specialized Venge S-Works frame/Zipp 858 wheels.

We compared those numbers to two different online bike calculators, setting them to 25 degrees Celsius, 50m elevation, 8kg bike weight, riding in the drops, and default values otherwise. Here are the results:

• 300 watts, 100kg rider
  Zwift Carbon, 32mm carbon wheels: 38kph
  Specialized Venge S-Works, Zipp 858 wheels: 38.9kph
  BikeCalculator.com: 39.16kph
  Kreuzotter.de result: 37.1kph
• 225 watts, 75kg rider
  Zwift Carbon, 32mm carbon wheels: 35.8kph
  Specialized Venge S-Works, Zipp 858 wheels: 37.2kph
  BikeCalculator.com: 35.77kph
  Kreuzotter.de result: 34.8kph
• 150 watts, 75kg rider
  Zwift Carbon, 32mm carbon wheels: 30.8kph
  Specialized Venge S-Works, Zipp 858 wheels: 31.4kph
  BikeCalculator.com: 30.55kph
  Kreuzotter.de result: 30kph
• 150 watts, 50kg rider
  Zwift Carbon, 32mm carbon wheels: 33.7kph
  Specialized Venge S-Works, Zipp 858 wheels: 34.4kph
  BikeCalculator.com: 31.44kph
  Kreuzotter.de result: 31.7kph

As you can see, Zwift speeds are generally within 2kph of the bike calculators, with the greatest difference being at the 150 watts/50kg rider test where the difference between one calculator and our fast test setup was 2.96kph.

It’s worth noting that 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 impact speed outdoors just like they do in Zwift!

Overall, I think it’s safe to say Zwift makes some slightly generous assumptions about your posture and environment which do not 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

Stopping at traffic lights, braking to avoid vehicles or other obstacles, slowing into turns on steep descents–these are all 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. And there’s coasting, which we do all the time outdoors, but rarely on Zwift.

Consequently, your Zwift average speeds will logically always be higher, even for similar effort levels.

Possibility #4: Zwift Gives You More Opportunities to Draft

Drafting in Zwift, like drafting outdoors, results in a power savings of ~25% or more. If you are comparing your outdoor solo ride speeds with Zwift speeds, the draft effect alone will make a noticeable difference since you often benefit from some draft effect 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.

Pro tip: if you’re really looking to take your fitness to the next level, start training with power and TSS, with less focus on distance. Here are two top books on this subject: