Your mileage may vary

(Ron Means) #1

I did a three lap ride last night and Zwift said 10.0 miles, I was careful to start my Garmin when I started my ride as well and it recorded 10.6 miles seems to be off quite a bit, I was wondering could this be tire size related maybe? I’m riding 700 x 25 and my Garmin knows that. Is anyone else seeing a difference like this? 

Thanks for an awesome experience so far, Ron 

(Eric C. (Zwift HQ)) #2

Hi Ron,

Your Garmin is recording distance as based on the speed of your tire. It does not take into account Zwift’s virtual world so the numbers will always be off.

(Ron Means) #3

So which is correct?

(Eric C. (Zwift HQ)) #4

Since you’re riding in Zwift? Ours. If you were riding outdoors? Your Garmin.

(Ron Means) #5

On my trainer, its simple math with the wheel size and how many times around = distance, so it seems to me the island is scaling it to the island miles maybe?

(Eric C. (Zwift HQ)) #6

The number of times your wheel revolves in real life doesn’t necessarily equate with the speed you’re going in our virtual environment. We also take things like wind resistance, road resistance, elevation, and more into account when calculating your in-game speed which, in turn, will affect your distance.

So, yes, while riding outdoors (in real life), the simple math of how many times your wheel rotates determining distance is very accurate.

But how often it rotates on your trainer doesn’t always equate in Zwift as all the factors are virtual on the island.

(Ron Means) #7

Thank You Eric C. For explaning this to me i do understand and can see how complicated it can be to try to make all the other influences apply fairly to everyone on the island, you are doing a great job and we appreciate it alot

(Eric C. (Zwift HQ)) #8

No problem, Ron! Totally understand that having all these various forms of measurement not necessarily adding up can be confusing :slight_smile:

(. Mr. Fantastic ODZ-ELab (C)) #9

If you want to see “accurate” distance between the island and your garmin, you will need to End your ride at the same place you started.  this (with my Kurt kennetic) is within about 2 tenths over a few laps  and YES your tire size does make a difference, but in the long run its not large,  the biggest difference is stopping where you started.

(# Leon Shaner(NEO)) #10

Hey, Eric, do you already have a request for enhancement logged to have Zwift “advertise” itself as an ANT+ speed/cadence sensor so that folks can pair that with whatever they normally track rides with outside, so that the numbers will match?

I expect the ANT+ dongle should be able to operate both in receive and transmit mode to allow the suggestion to work, since you are able to control “smart” trainers over ANT+, right?

Meanwhile, if not possible to do it “generically” for everyone using the ANT+ dongle, what about something “smart” trainer-specific, as in advertise to the trainer what the in-game speed is, so that the trainer can re-transmit a corresponding speed over ANT+, based on a certain wheel size (which the user would enter in their GPS for tracking purposes).

(nhat tran) #11

A mile is a mile.   There can’t be a Zwift mile that is different than a wheel mile.   

I noticed that my Zwift mile is ALWAYS higher than my wheel mile.   Similarly, my Zwift avg MPH is ALWAYS higher than my wheel avg MPH.   And curiously enough, the ratio of Zwift Mile / Wheel Mile is exactly the same ratio as Zwift MPH / wheel MPH.   

Regardless, I don’t understand why Zwift has to do “calculated” mile.   Why can’t Zwift measure the actual mile and vary the trainer resistance base on ascend, descend, wind resistance, drafting, etc.?   In other words, all Zwift has to do is to calculate the resistance for the trainer (period).   I think Zwift currently calculates the mileage, speed, and the trainer resistance.   And as a result, things get muddy and not accurate.   No wonder everyone in my bike club laughs at my avg MPH on Zwift as fantasy.

I wish Zwift can fix this.   


(Steve Ellis) #12

Per the Speed & Power calculator at


  • a road bike,
  • hands on the drops,
  • rider: 172 cm
  • rider: 71,3 kg
  • bicycle: 9.5 kg
  • temperature:20 °C
  • altitude: 350 m
  • grade:: 0%
  • wind: 0 kph
  • tires: road

pedaling at 100 watts, speed is 26 kph

pedaling at 200 watts, speed is 33.8 kph

Scenario A: If Zwift and a Zwifter use the same assumptions, and had 34 kilometers of Zwift flat roads, pedaling against 100 watts of resistance, in 1 hour Zwift would show 26 kilometers traveled.

Scenario B: In the same conditions, but the trainer is set to provide 200 watts of resistance, and the Zwifter is pedaling in the same gear on the trainer bike, at the same cadence, after 1 hour Zwift would show 33.8 kilometers traveled – but the bike wheel on the indoor trainer would have gone around the same number of times as in Scenario A. 

Indoor trainer wheel revolutions and virtual world distance are not the same.

And that’s just in the simplest conditions of constantly flat road. Take the scenario of coasting down Alpe du Zwift. That’ll last for minutes. In Zwift you’ll travel kilometers. Indoors, your wheel isn’t going around at all. It’s still. But coasting down a mountain is a realistic simulation of outdoor cycling. 

In between flat roads and a mountain descent are other conditions where a trainer wheel revolution and a virtual world distance are not the same.