One popular question from wheel-on trainer users in the Zwift Riders Facebook group is
“Why don’t my Garmin computer’s distance/speed numbers match Zwift’s?”
The answer to this is quite simple, although it can be hard to wrap your head around if you’re new to virtual cycling.
Simply put, Zwift calculates speed (and therefore distance) based on power, while your Garmin calculates speed based on wheel revolutions. Because of this, your numbers will never match. And that’s OK!
More Details
Let’s imagine you have your Garmin turned on while Zwifting. Here’s how distance computation is happening on your Garmin and Zwift:
- Your Garmin is picking up each wheel revolution via the speed sensor on your back wheel and multiplying the number of revolutions by your tire circumference to compute speed and distance.
- Zwift is taking the power you’re putting into the pedals and feeding that into its algorithm to determine how fast you go in the Zwift environment given your weight, height, virtual equipment, current road incline, drafting status, and more.
Note: if you don’t have a smart trainer or power meter, Zwift is estimating your power based on known power curves for particular trainers (we call this “zPower” or “virtual power”). In this case, Zwift does use your speed sensor, but it is converting that wheel revolution data into power numbers. Read more about Zwift’s virtual power calculations >
Mind *blown*!
I see the same thing happening with a kickr core. Social ride recorded in Zwift is 55km, same ride according to Garmin watch 45km. Power numbers and duration are if course the same. avg speed and total distance differ. Issue can’t be wheel revolutions witj the kickr core, can it? Same sensors are connected to both. Probably the drafting effect in Zwift is not picked up on the Garmin?
My Garmin 945 used to be about 15% slower on average compared to zwift speed depending on the route. Last few rides it’s now saying 60%-70% slower or 8mph-10mph. Zwift is still fine and the power is accurate. I don’t get what happened. Haha. Time to reset it and see if I can fix it
Same for Neo 2T and 945. (Not sure it is 15% all the time but it is slower). It is slower if I use 100% trainer effect or not. I guess the main question is how Tacx (or any other company) calculates the speed it transmit to the cycling computer. I can’t find the answer to it.
“while your Garmin calculates speed based on wheel revolutions”
But in direct drive there is no wheel and in smart trainer the speed is taken from the trainer not the wheel (and today many do not have wheel based speedometer and just use the GPS).
So why there is still a (BIG) difference? How the trainer calculates the speed. For example Neo has the data on rider and bicycle weight and it generates the load (sloop) and calculates the power. So calculated and transmitted speed can be very similar to Zwift w/o maybe drafting effect.
Garmin’s speed correlates with the cadence,while Zwift’s speed correlates with the power.
At the end of the day, Zwift is a game. All I know is I ride my bike loads more than I would outside. I do more miles and burn more calories no matter how you wish to measure it on Zwift compared to outdoors. I only free ride so far and I love it.