speed is way out when riding

(Lee Hathaway) #1

hi,when i pair my tacx blue motion to zwift my speed on zwift is the same as on my garmin,but when i start to ride the speed is totally different.i can be doing 20 mph but zwift will show 8mph,then going downhill zwift is alot faster than i’m actually going.i also loose the power quite alot too.i’ve tried having my speed sensor close to the ant+ dongle but still does not work when riding,as i’ve said everything is fine on the start up screen (pairing devices) but is completly wrong when riding.i hope somebody can help as this was my first go on zwift


(Kyle Polansky) #2

Zwift does not use your trainer speed to determine in-game speed. It uses your current power output and weight to determine in-game speed. So for example, if you have a “dumb” trainer going the same “real” speed (or power output), you will go uphill much slower than downhill in-game.


Hope this helps, also visit the official Zwift answer in the FAQ ( https://support.zwift.com/hc/en-us/articles/204866355-Frequently-Asked-Questions) under “Why is the speed/distance on my bike computer different from in Zwift?”


(Lee Hathaway) #3

thanks for your reply,so i need to forget about the speed and look at the watts more,will this make it harder to try and improve my times if the speed is out though.i was getting frustrated with the fact that i would be pedaling as fast as i could when i started a kom and i would end up getting no way near


(Kyle Polansky) #4

It’s even easier to compare efforts with a power meter! You should still be able to compare segment speeds too. Pacing will be a little more important (putting out more power on steeper parts and going easier on the flat/downhill sections will make you the fastest in Zwift). It won’t be perfect if you are using zPower, but should give you a pretty good idea, especially over longer segments. If you really want your trainer speed, you could always record on your Garmin and analyse that afterwards.


(Lee Hathaway) #5

thanks for that i will try again tomorrow morning


(Darryl Keller) #6

I found the speeds laughable when using my Kinetic paired with Zwift.  5 MPH when my garmin is showing 12.  Zwift is no fun when people pass you constantly, no matter how hard you work.   FYI to Zwift–THAT’S the reason we’re using you, for in-game experience!  Otherwise we’ll just use a better training program if numbers is all we want.


(Cathy McDonald) #7

I’m really surprised that Zwift doesn’t have a spot in the settings where you can put in your tire size of your bike.  This would fix the speed/distance inaccuracies.  This is how a Garmin determines your speed/distance when you can’t use a satellite.  Since Zwift is for indoors, a tire size is crucial.  I really hope this changes.  So all in all, go by your watch if you have one for speed/distance accuracies.


(Randall Lupien (VV)) #8

I just got Zwift up and running tonight. I noticed the speed issue right away. If I can’t figure out a workaround of some sort I suspect this will be a deal breaker for me. I’ll play with it for the trial period and dump it after if I can’t find a way around the speed issue. If I’m going to be riding by myself I’ll just move to CycleOps.



(Randall Lupien (VV)) #9

Well, I have a workaround (modifying height and weight settings) that seems to resolve the speed delta issue for the most part. Although, Zwift now thinks I’m 9’11” tall (only because that’s the upper limit) and weigh 95 lbs. Those settings seemed to get the delta down to ~1-2 mph with some jitter. Sometimes Zwift/Garmin actually matched. Although, I think having Zwift set up like this makes it nothing more than entertainment (at best). I don’t think any of the data coming from the app would be valid. Now it would be nice if they put some goofy stuff on the side of the road to look at. I plan to play with these numbers more and see what I can come up with. I wonder what it will do if it thinks I’m say, 1’2” and 340 lbs. (cross posted). 


(Marty Martinez) #10

Agree with the others that say the speed metric needs some refining. I KNOW that using the gearing and cadence that I’m using in game should have me moving at 14-16 mph. Zwift says I’m going 4-9 mph. Not even close.

Zwift using power to compute speed. And it’s completely wrong. I tried to resolve the problem via chat. No luck. I’m dumping Zwift and applying the $120/ year to another program.


(Randall Lupien (VV)) #11

Here’s my follow up to my comment. While a power input is not required to ride Zwift, it is necessary if you want data validity. I first tried a CycleOps PowerCal. It’s a chest strap heart monitor that also outputs power based on heart rate. It’s relatively inexpensive (about $100 US) and it does interact with Zwift well. Although, because your power output is based on heart rate, it does take some time for power output to ramp. That means, for any competitive power ramp needed (sprints), heart rate needs to be ramped first. This means, if you want to contest a sprint, you have to launch before everyone else in order to be in a competitive power zone when everyone else launches. But, the PowerCal does work well with steady efforts (group rides). The complete solution to this issues is an actual power meter. This resolved all my Zwift/power issues. All my numbers between Zwift/Garmin just about match up. There is still a bit of a speed delta between the two but, I would expect that. I disregard the Garmin speed and default to Zwift. I figure if I can stay in the peleton, all is good.



(Marty Martinez) #12

My comment may have made it seem like I am not training with power. I do have power. A Tacx Neo and Stages. The power numbers on Zwift roughly match my other readouts, but for whatever reason the speed is always way way off.

Others can disagree and say that the power/ speed algorithms are correct. The fact remains though, that if I’m pushing a gear at 90+ RPM that would have me moving at 16mph if I were outside, then that’s what it should show in the game. Maybe knock 1 or 2 mph off to account for the lack of wind resistance indoors if you really want to get picky. 

Said another way, if a given power number is enough to propel the bike to 16mph in real life, then that is how fast it should show in game. Wind resistance would be a greater factor the faster you are moving obviously, but at a modest 16mph it’s not going to be much. 


(Randall Lupien (VV)) #13

Humm, I do see a speed delta and I expect it at some points (down hill, etc.). But, on the flats Zwift tends to show my speed as slightly higher than the Garmin. You’re sound like a question for tech support. Something’s not right. :frowning:


(Marty Martinez) #14

That’s what I was thinking as well, Randall. I appreciate you taking the time to respond and help troubleshoot. I tried to get a solution using chat here. I was told that the game and my computer will, “Never match.”  I wasn’t expecting them to literally match, just be roughly the same. I was then told that the Zwift numbers are, “Very accurate.” So if it says I’m going 4mph then that’s the end of that. Kinda funny since I’d have to have a massive cassette to do 4mph while at a 90+ cadence. 

 Nothing else left to do other than live with it or bail. 


(Kyle Polansky) #15

Have you made sure that your rider weight is correct? Also, have you made sure that the grade of the virtual road is what you expect? For example, if you are going up a hill, I would expect 4mph in game from “normal” speeds. If you are going down a hill, you might be going 30mph from the same power.


(Randall Lupien (VV)) #16

Yeah, if you’re at 4mph at 90rpms on the flats, something’s wrong for sure. And, if you’re at 4mph while climbing like 5-8%, something’s wrong as well (unless you’re a really slow climber. And, I kind of don’t get that impression). In Zwift at ~2.5w/kg at ~90rpm I show ~21-22 for speed. The Garmin will show about 16-18 at the same time. I’ll be dong a rain ride on Zwift this Sat. I’ll track all my numbers and post them here. By the way, my bio data is 5’8"/160lbs/56 years old.


(Russell Gowers [CRANKS] KISS (C)) #17

There seems to be a fundamental issue in that Zwift is trying to derive your speed from the power, rather than use the actual inputs from the speed sensor and increase the resistance of the turbo to compensate.

Assuming there is some resistance (ie not a steep downhill) there is a fixed relationship between your cadence and the speed of your rear wheel. So pedalling at 90rpm in 50/17 (for example) should always equate to about 20mph, whether on the flat or on a 10% gradient. The issue is whether I have enough power to turn the pedals at 90rpm in 50/17 on a 10% gradient (I definitely don’t) - so Zwift should increase the resistance provided by the turbo to simulate how difficult this would be. The resistance should be even greater assuming I’m a fat bazzer (which I am). BUT, if I’m capable of maintaining the cadence, the speed shouldn’t change.


(Marty Martinez) #18

Thanks Randall.

Exactly Russell, that is what I was trying to get across to chat support. When I’m getting 5 mph on a flat when I know that a particular gear/cadence combo yields three times that in real world riding, it is frustrating. I’m beginning to sound like a broken record so I’ll stop. I suspect that support and devs are already well aware of the issue.


(Randall Lupien (VV)) #19

Hi Marty, Ok, the data from today’s ride shows that power, heart rate and cadence all just about match between Zwift/Garmin (very little to no delta). These numbers do jump around at little but, are fairly stable. The speed is a little different. I Looked at mph on the flats and climbing. I do see a bit of a delta but, nothing like you’re seeing. I think the most divergence I saw was about 6mph. That is, if the Garmin said I was going 15mph, Zwift was calling it 21mph. Zwift is always higher (yay!). And, there are times when Garmin/Zwift are in sync. Anyway, something is not right for you and I’m hoping tech support will dig into this. I posted a video to Youtube so you can see my actual data for validation. It’s a little jumpy because I’m on rollers. https://www.youtube.com/watch?v=yb5gMcw3vo0


(J DeL... IMLP IMFL) #20

I’m having the same issues as Marty.  I just finished my 14 day trial and I was shocked at how wildly the speed swung on my setup.  I have a new Wahoo Kickr Snap.  I’ve had a Wahoo speed/cadence sensor on my bike for about a year.  I have a Garmin Vivoactive and I do wear a Garmin HR strap. Everything seems to work well except for the speed in Zwift.  Just a slight incline and my speed drops a great deal. Climbing hills, my rider stops and starts I’m going so slow but I’m spinning away fast on my actual bike (albeit in a low gear).  I’m not expecting perfection by any means.  But the speeds swings are WAY too large for me.  Plus and minus 8 mph when riding essentially level.