One popular question in the Zwift Riders Facebook group is
“Why do my Garmin distance/speed numbers not match Zwift’s?”
The answer to this is quite simple, although it can be hard to wrap your head around if you aren’t used to virtual cycling.
Simply put, Zwift calculates your speed (and therefore your distance) based on power, while your Garmin calculates your distance 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 what is happening in terms of distance computation on your Garmin and Zwift:
- Your Garmin is picking up each wheel revolution via its speed sensor and multiplying the number of revolutions by your tire circumference to compute speed and distance traveled.
- Zwift is taking the power you’re putting into the pedals and feeding that into its algorithm to determine how far you would travel in the Zwift environment given your weight, aerodynamics, current road incline, drafting, 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”). In this case, Zwift does use your speed sensor, but it is converting that wheel revolution data into power numbers.
So I gather using a real PM (and speed sensor) in conjunction with a smart-trainer will really mess up Zwift? I have an Elite smart-trainer and I was thinking if I attach a real PM (vs power estimator), cadence sensor and speed sensor, but use the resistance of the smart-trainer, I would get more accurate real-world numbers.
Many people do this–I do, in fact. Using an actual PM lets you get the same power readings indoors and out, which is helpful for training. But you don’t need a speed sensor indoors… that’s only needed if you don’t have a PM or smart trainer.