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. Read more about Zwift’s virtual power calculations >
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.
I just finished setting up the my classic fluid trainer with a cadence/speed sensor and then did my first ride with zwift. I enjoyed it however I was concerned with the labile speeds that zwift recorded. I was maintaining 14mph throughout the course but zwift would calculate 4mph and then speed up to 22mph and then back down to 4mph. It did not seem to be anywhere close to what I was actually doing. Any suggestions would be helpful.
Hello, i connected my Edge 1030 to Tacx Neo. the higher the speed the more difference i see. For example Zwift says 43Km Edge only has 34Km. Power and cadance are (almost) equal…Heartrate also only speed does not match. Can set wheel size manual but it does not solve it…
Of course they don’t match! Did you not read the post above?
But which is closer to matching what the actual distance would be outdoors?
Zwift, for sure. It may be a little generous, but it’s close! https://zwiftinsider.com/zwift-speeds/
Strike that last question.