Very large speed difference from trainer and zwift

Since I started using zwift. The speed difference from what I am really putting out and what zwift sees is very very different. I use a garmin to monitor my true speed and use a computrainer for my training. I have let my computrainer calculate my speed and I have also used a speed sensor (wahoo)connected to zwift. And the results are the same. Zwift is either says I’m going very slow or very fast. I could handle a one or two mph difference. But when I am holding 20 mph on a climb and zwift decides that I should be doing three. At the other end I could be doing 17 but zwift says +30 That’s a big problem. Has anyone else had this? What is the patch?

If I continue to get such poor results. I will not be giving my money to such a poor product.

Keep in mind that your Zwift speed is based on the amount of power you are putting out, not how fast your wheel is turning. Also, be sure your trainer is properly set up in Zwift, as well as your height and weight. Additionally, if your ‘Difficulty’ slider isn’t set at 100% your Zwift speed will hardly ever match your wheel speed, even on a smart trainer.

I know that my weight and height is correct. At one point I was at over 250 watts and was only going 3 MPH. I was grinding on it pretty hard( At least for what my legs could do) just to see what would happen.

The difficulty slider. Honestly I didn’t know what that was. I will max it out to 100 the next time i get on the bike. See if that helps. Thanks for the help Nigel Tufnel…

Depending on the gradient of the climb, 3 mph for 250 watts is certainly a possibility.

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I guess my issue is that if my garmin is reading 20(yes it’s setup correctly). How does that translate to three on zwift. It gives me the impression that zwift is cheating me. I’m spinning and the wheel is going like a bat out of ■■■■ and zwift say im going 3.

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This is how Zwift speed is determined:

Zwift takes the wheel speed of your bike and the power curve of the trainer selected on the Pairing screen and converts it to watts. With those watts, the weight entered, height entered, in-game bike used, in-game drafting (Tri and TT bikes get no drafting boost), in-game wheel set, in-game road surface, and in-game virtual elevation changes your in-game speed is determined.

The speed of your Garmin and Zwift will hardly ever match, the Garmin knows nothing of the in game variables.


Explain to me then how My speed, my watts, my cadence, has not changed in any way in the real world. But Zwift says I am going very very slow. I don’t get it. I am not going slower in the real world but swift say I am going slow. If this happened to everyone nobody would use it because it’s not accurate.

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Without knowing your exact set up I can’t be too specific on this, it think about it this way (and keep in mind that I am presuming you are not riding a smart trainer):

If you are on a flat road in a certain gear at a certain cadence in real life or in Zwift you would be traveling at a certain speed and putting out a certain amount of watts.

In real life, If you then hit a climb and continue in the same gear at the same cadence your speed would remain the same, but the amount of watts needed to maintain that speed would increase.

Using a ‘dumb’ trainer in Zwift, however, If you hit a climb and maintain the same cadence your wheel (Garmin) speed would remain the same, as would your watts. This, in turn, would translate to a slower in-game speed, even though your Garmin would say your going the same speed. And as the gradient increases the difference between wheel speed and in-game speed will get bigger. In order to keep your speed up as the gradient increases you would need to put out more watts (just like in real life), but on a dumb trainer this still means going faster, in terms of wheel speed.

I hope that makes some sense. This, to me, is one of the benefits of using a smart trainer with Zwift. Then the resistance increases as the gradient increases, so it’s more of a ‘real world’ feel.


Is a computrainer smart or dumb? Because from what I can tell the racermate software seens as smart as it get’s.

Look I Feel like things are getting defensive. That’s not what I want. I’m just looking for answers. Maybe it’s a setup on my end I don’t know. All I know is my racermate software matches my garmin. I calibrate before every use. I just want it to translate to zwift is all.

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According to this page the Racermate Computrainer is a smart trainer but it requires a FTDI USB to stereo adapter.

This video explains:

Thank you for your reply. I have everything connected and running properly. computrainer and racermate working well.

I can’t say that I truly know whether a Computrainer is ‘smart’ or ‘dumb’, so here’s a question: When you start going up a hill in Zwift, does the resistance on the trainer automatically increase to make it harder to pedal at the same cadence without having to put out more watts? (This is presuming that your difficulty slider is set far enough to the right to make a noticeable difference.)

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Please reread my post, there is no way your Garmin or the other software knows about the Zwift in game variables.

There can be massive speed difference between the mountain bike and the TT bikes within Zwift.

Not getting defensive, just stating facts.


Hi Kevin. I get that you are frustrated that your real world isn’t your Zwift world with the result you are seeing different speeds but it doesn’t matter. I’m not technical enough to explain why but just go with the feel (i.e. the hills are harder) and you will get so much from Zwift.

Remember it’s the same for all of us and if you and I rode together on a real course and on a Zwift virtual course I’m willing to bet I would feel just as tired keeping up with you in both. That’s of course if I could keep up with you, which is another point why Zwift speed isn’t the same.

I think it delivers more; if in the real world I ride 100% and you rode say only 80% but were much fitter than me I’d be beaten every time. In Zwift it would be the reverse. Now when both of us give 100% then it gets real fun and who cares it the “speed” shows 1 or 100 it will be all about which of us get the line first and that applies to both worlds.

One of the problems for Zwift is that have done such a good job making Zwift seem real that many people expect more than Zwift can deliver, see lots of discussions about racing and cheating.

Under controlled conditions it is possible to use Zwift for very real competition but until everything can be certified there would be nothing stopping me for example using an eBike and beating

One of the real world rides I do I looked at the KoM time (mine placed 1000 ish BTW) I wondered what it would take to beat it so rode on my motorbike and I couldn’t get the KOM time in Strava. The result of that was I gave up worrying about if the stats were real or not. Instead I use the stats from both to monitor my personal gains (or loss), but what I don’t do is compare Zwift to Strava.

Hope that helps, and just aim for four in Zwift rather and three knowing that virtual or real world you are going faster.

The garmin is just a reference to what my bike actually doing. I know that there is no connectivity of the garmin to zwift. All I am saying is that zwift seems to think that if I climb a hill and I don’t change actual speed or cadence or watts. Zwift seems to think I should be be doing 3 mph.

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Shouldn’t a cyclist be moving at a slower speed while going up hill? Assuming constant watts.


@Kevin_Vanveelen zwift doesn’t use a speed metric from the trainer. It takes your power output to determine speed based on the terrain in the game (along with weight, air resistance, rolling resistance, etc).

So in your example you are going 20mph on the flats at 200W (just random numbers I’m using) then hit a 5% grade but still maintain 200W the laws of physics demand you go slower. To keep the same speed you would need to increase your power output.

That sounds correct to me. Again reread my post on how speed is calculated within Zwift. You will slow dow while going up a virtual incline and speed up while going down.

it sounds like you’re just misunderstanding how zwift functions which isn’t being helped because your trainer isn’t communicating to your legs what the world in zwift is doing

are your mysteriously slow speeds only on uphills? and your mysteriously fast speeds only on downhills?
that’d tell you the answer right there.

That might be your problem right there. Zwift uses either a power or speed sensor to determine your power output. If you really do have a speed sensor connected to Zwift, the Computrainer connection might be doing nothing at all.

That being said, the variations you are seeing are indeed probably due to uphill and downhill gradients (3 mph sounds nothing out of the ordinary for many of the steeper climbs). The gradient can be a bit difficult to tell from the scenery alone, so take a look at the percentage in the top right corner the next time you think this is happening.