Speed display too low

(E nricoGimondi ZHR (G)) #1

I have a traditional trainer (Blackburn) and am using Garmin speed and cadence sensors.

On the computer unit mounted on my stem (an Edge 25), I’ll see, for example, an cadence of 90 and a speed of 20 mph.  

But during a ride on Zwift, although the cadence matches my Edge, but speed will display as about 4 mph.

I’m really not that slow!

The fact that my bike mounted computer shows a reasonable speed for the gearing and cadence suggests that the wheel isn’t and the hub mounted speed sensor isn’t slipping either.

Also, the sensors are both working and sending accurate readings to my bike computer.  And the USB ANT+ are apparently doing their jobs (ANT+ is sitting under the bike close to both sensors)

So … any ideas why the Zwift online display of my speed is slow low?



(Paul Allen (Watopia Wayfinder)) #2

Zwift is a virtual world with hills and mountains. When you are going up one of the hills I would expect the speed to drop sometimes down to 4mph. Zwift uses your rear wheel speed and the know power curve of the trainer to calculate power and translates that to speed base on the virtual terrain in Zwift.

For the most part ignore the speed that your bike computer is telling.

(E nricoGimondi ZHR (G)) #3

Paul -

thanks.   But it is not a matter of my speed dropping on hills. Even at the very beginning of a ride on apparently flat terrain, I can be pedaling at a reasonable cadence (eg  90 -110) and in a mid-range gear a still never get above 4-6 mph.  If I sprint, I might get it to 10 mph.   

I am in reasonably good shape and ride a lot in the real world so this is not the case of someone being shocked at the effort it takes to ride more than 4 mph.

I reference the speed shown on my computer because I think that it is a pretty good approximation of what my speed would be at that gear and cadence.  


(Paul Allen (Watopia Wayfinder)) #4

What size wheel are you using?

(E nricoGimondi ZHR (G)) #5

It’s 27x 1 1/8

(Paul Allen (Watopia Wayfinder)) #6

Zwift only supports 700 wheels right now so the difference in wheel size could be part of the issue.

also make sure the Ant+ dongle is right under the bike if possible. I use an USB extension to get it there.

(Paul Allen (Watopia Wayfinder)) #7

Another this I need to ask is what trainer are you selecting within Zwift.

(E nricoGimondi ZHR (G)) #8

Paul -

thanks again for helping me trouble-shoot this.  I have some 700c’s on a different bike. I’ll put those on and see how that works. Or just mount that bike on the trainer.

I do have the ANT on an extension and it is very close to the rear wheel.

I am using a Blackburn Trackstand Ultra.  It’s listed as a supported trainer.  I also tried selecting “not listed” as the trainer to see if that would help, but no luck. Still showed about 4 mph 

(E nricoGimondi ZHR (G)) #9

Switched bikes on the trainer to one with 700c’s; pumped tires to  correct pressure;left the cadence sensor off and in a different room, just to remove that from the equation.  Speed is now up to about 12-15 mph, so that’s moving in the right direction. Put cadence sensor on and no change, so it wasn’t an interference issue.

  I still think Zwift is under-reporting my speed, based on cadence and gearing.

(Michael Shirey) #10

Is your weight correct in Zwift?

(Brian Field) #11

Same problem on Tacx Flow, flat is ok but hills are way off 250wat and 2mph, but yet others can come past hitting double figures, resistance is a no go either its just like riding a fluid trainer

(E nricoGimondi ZHR (G)) #12

“Is your weight correct in Zwift?”

Yes.  Though that shouldn’t matter on the flats should it? 

Speed should be a function of the rotational speed of the rear wheel

(Paul Allen (Watopia Wayfinder)) #13

And the selected trainer within Zwift.

(E nricoGimondi ZHR (G)) #14

Understood.  But assuming I have selected the correct trainer, the speed shouldn’t  be off due to an inaccurate weight entry.  Right? 

I have selected the correct trainer, yet the speed display reads low. If I were to lose weight, but pedal at the same cadence in the same gear, the rear wheel would rotate at the same rate, so display speed should remain the same.  (Assuming that gradient is the same too since I understand Zwift takes that into account).

(Lee Woodcock) #15

I had the same issue today using a Tacx Flow, it seemed OK last week but the speed was definitely recoding low today. I did the same route, 16km travelled last week and 8km, went easier today but not that much.

(Paul Allen (Watopia Wayfinder)) #16


Compare the watts, not the speed. We’re the watts similar between the 2 rides?

(Lee Woodcock) #17

Hi Paul, 22w difference over a 30 min ride.

(Paul Allen (Watopia Wayfinder)) #18


fitness, fatigue, calibration or combination of them could cause the difference in speed and feeling.

(Lee Woodcock) #19

Hi Paul, agreed but also know what it’s like when you are pedalling hard and it is displaying 4 or 5 kph.  

(adam sewall) #20

Same issue - Garmin 920 with Garmin sensors - 700CC wheel set, classic trainer set up, and when climbing the speed shows 4mph - same course the other week - was climbing at 12-15mph - So the 920 shows the same relative speed when using Zwift last week but now at 4mph.

Not sure how to resolve the issue.

Interference - no

Weight - correct

Tire size - 700CC