How is Zwift calculating speed

I just got a Tacx Flux 2 and am starting to use Zwift. When I pair, if I pick ‘speed sensor’ it wants me to enter the tyre size, which I’m guessing would be the size of the tire that would be on my bike if it wasn’t on a smart trainer. But I can only pair the Flux under ‘power meter’ and it also pairs with cadenced and speed sensors. What I want to know is how is Zwift calculating my speed, considering I’m on a mountain bike with a smaller wheel than a road bike. I don’t want the speed a certain power translates to be different on Zwift that it will be on the road.

Pair your trainer as power and read this: How Does Zwift Determine My Speed?

Cool. So having picked the MTB frame in Zwift (and wheels) it knows. Cool thanks.


Zwift calculates speed based on power and in-game variables.

No, that has no correlation to your pairing of trainers and sensors. Btw, the MTB frame in Zwift is the slowest on pavement and faster only on dirt surfaces like the jungle in Watopia.

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Pair as power and controllable, not speed.

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Yes, but having read the link you posted, one of those variables is the frame - mine being Mountain bike. Which Zwift says goes slower for the same power. But faster off road.

You maybe confusing the frame chosen In Game and the actual frame you are riding.

The actual frame you are riding has nothing to do with your speed in Zwift.
The Game does not care what kind of bike you are physically sitting on.
The actual bike you are on is merely the device you use to generate Watts.
Some people will use their actual bike in order to stay accustomed to it, other people like to use a different frame that is more comfortable.

You should look at the style riding you do and compare it to riding Zwift.
Zwift riding is still TRAINER riding.
It tends to be more seated and a less aero posture.
I chose not to use my road bike because I wanted a more comfortable seat and a more upright posture.
When I mountain bike, I stand up on the pedals a lot if not most the time.
This is the most unlike Zwift riding that can be.
A MTB on zwift is usually in the highest gear available.
Most people almost never use that gear IRL outside.
That demonstrates how different IRL MTB riding is from Zwift.

Zwift is not like road riding or MTB.
It is a similar but a very different discipline.

Don’t worry about matching you real bike to your Zwift bike.
Also, you have a smart trainer so no wheel size is needed.

Yeh. I wasn’t confused. The reason for choosing mtb (which is the kind of bike I have) is that it means if I can cycle 30 miles in Zwift, it’ll be the closest match to what it’ll be for me to cycle 30 miles in real world. Choosing the MTB frame in Zwift means that my Zwift power/speed will be closer to my real world power/speed.

That is an incorrect assumption.

In Zwift you don’t have to worry about slowing down for corners, stop signs, cars and anything else. Your pedaling will also be a little different since you cannot rock the bike (unless you have a rocker plate).

Within Zwift it is about effort (power or HR) over time.

Again, MTB in real life is completely different.
30 miles on a MTB (I’m assuming cross country) takes me 2.5 - 3 hours and is spent standing up a lot of the time.
Zwifting tends to be mostly seated.

If you want to get accustomed to your frame, fine, put your MTB on your smart trainer.
Your MTB gearing is going to be very low for use on Zwift unless you put a small road group cassett on your trainer.
I would recommend that you place your smart trainer on the 100% setting and go ride.

Your avatar can be on any bike.

Yeh I have a mountain bike but I don’t cycle off road :slight_smile: My partner has a road bike so eventually I want to be cycling in real world with her. So am looking for Zwift to be giving me a relatively accurate indication of how far I can go. If I choose a road bik in Zwift I’ll be working the same amount. It won’t change how much power I put in or how much of a work out I’m getting but I’ll be able to do more miles faster.

You cannot compare Zwift speed and distance to real world speed and distance.

Hi Gordon,
I see where you are going with this.
Another idea for you would be to find out from your partner how long she rides for and try to replicate that. ie if her rides are generally 1-2 hours make that your goal.
If she is able to let you know her perceived rate of exertion (or heart rate info) this would also be of use to you.
I say this as Zwift miles and outdoor miles are generally not the same, which ever Zwift bike you use.
Hope that may help.

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Even when my in-game and real world hardware matches and there is no wind, I probably can’t get real speeds as high as in-game for the same power on flat roads.

Zwift must calculate using some very aero riding position

Ok guys i know I’m not going to get an accurate match. However, I know for certain that if I pick a road bike on Zwift, I’ll be going A LOT faster than I can go on my MTB in the real world. And to be clear, I mean on roads. :). Thanks for all the info/comments, I got the answer I wanted.