Neo Speed and Power not right

(Antony Zaddo) #1

First day with zwift and a smart trainer, trying to get things setup.  

I am not an overly serious rider, my bike trainer failed and I am replacing it with a tacx neo.   I have attached my 20 year old hybrid steel framed bike with 21 gears (don’t laugh, I just want something to provide resistance and make it fun)

In top gear doing 60rpm, I am going 26 kpm.  On the road I would be going about 45kph.

Therefore zwift is not setting the right power level on the neo.

I don’t see how zwift can be getting this wrong, because the rpm’s of the flywheel should match what my bike does when it is on the road.

The only thing I can think is the wheel diameter used for speed calc’s is wrong.  But I can’t find any setting for this.

I can’t find any settings to adjust this.  Please help.


(Eric C. (Zwift HQ)) #2

Hi Antony,

Thanks for writing. Just an FYI: the virtual speed that we show within Zwift is based on the power being generated on the Neo and virtual factors (such as the wheels you’ve picked, the frame you’ve picked, incline/decline, weight, cda, and road resistance).

Since the Tacx Neo requires zero calibration, we take the data that it is telling *us* and creating a speed based on that. We don’t actually communicate to the Neo aside from changing its resistance based on the grade within game.

That being said, your real-world speed and Zwift’s will never quite match up because of these virtual factors.

(Steve Millier Team ZNZ) #3

I have the same issue too - Using a Tacx Neo the Zwift speed reading is way off actual road speed - please advise if there is a ‘fix’ to this even if it means ‘fixing’ the input data to provide a more realistic output set of results?


(Paul Allen) #4


Please explain in detail how the Zwift speed is off.

Zwift takes the power being broadcast by your Neo, your weight, your in-game bike, elevation changes and drafting to determine your in-game speed. 

(Steve Millier Team ZNZ) #5

Hi Paul an example is going up a 15% gradient at 95cadence in the big chainring and in a 13 rear and doing 5kph according to Zwift readout? Very odd knowing also when I ride my bike on the road the 53/13 will produce speeds of circa 40-45kph at 100-110 cadence on Zwift I am in the mid/high twenties?

Hope this helps?

Many thanks

(Paul Allen) #6


It’s about watts, not what gear you are in. The only way you can compare Zwift rides with IRL rides is by using the same power meter, meaning watts. Trying to compare them using gearing and cadence is close to impossible.

(Antony Zaddo) #7

There is definitely something off about how zwift calculates speed.  I resolved my issue by adjusting my weight setting.

I unfortunately got myself in a situation where I weigh 120kg.  If I set zwift to this weight, I run out of gears.  With cadence 110, top gear, my max speed is 26kph, on flats or going down hill.

When I changed my weight to 90kg, I get more realistic speeds. 

The cadence and the gear, drive the flywheel RPMs which should set speed.  Zwift should adjust power on the neo to match what is required to achieve this.

But for some reason, this is not how Zwift works.  I get vastly different speeds for different weight settings, even when going downhill.  When going downhill, weight should not be a significant factor as gravity will start to work for you.

I am a software developer and I have written my own test rig that calculates power input and speed output for the neo.   The results are in my opinion reasonably close to riding on the road.  This math isn’t hard, so Zwift have some hacks or bugs in their code that get it very wrong under some circumstances.

(Lav TIS Pacing) #8

Same for me. In the flat I cannot keep up with the group. I own bepro pedals and stages powermeter. My Neo/Zwift CP Curve is consistently 50W below the other power meters. Frustrating…

What’s the solution here?

(Andre Hufschmid) #9

Nope, i totally disagree. I’ve been using a Neo the past 2 years and its absolute accurate. The Zwift in World Speed is also almost spot on, as a matter of fact you’ll be even a bit faster because you don’t have the head winds in Zwift.

Secondly you should always pay attention to the gradient you’re currently on.


 I have the same problem with my tacx Neo and zwift, it marks like 50w less than Garmin power meter!! so what i think that we can do its what Antony Zaddo do, of your weight practically remove 1/4 of the weight to enter your training zone; until swift change their codes.


(Paul Allen) #11


It seems the logical solution to your issue would be to use the Garmin power meter as a power source and the Neo as a controllable trainer instead of dropping your weight.

This is not a Zwift issue, it is either a poorly calibrated trainer, a trainer that is not calibrated at all or the trainer could need repairs or a firmware update. Zwift does nothing to the watts being broadcast by the trainer.