Speed on Zwift slower than bike computer?

Have moovit speed and cadence sensors on dumb trainer. Both paired with Zwift and with bike computer. Bike computer shows consistent 25kph which feels about right compared with cycling outside for effort levels. But Zwift shows anything from 5kph to 39kph almost randomly. Any ideas what I’m doing wrong?

Your bike computer is probably picking the speed up from the speed sensor on your bike which does actually measure speed the same indoors as when you’re riding outside, it’s based on the speed that the wheel rotates, zwift uses some fancy formulae that includes weight and other things that have nothing whatsoever to do with speed and chucks some figure out, more details can be found here:

Your bike computer and dumb trainer don’t react to changes in elevation in Zwift. If you go uphill the avatar will slow down, your trainer wont change resistance like a smart trainer would, so you might just keep the exact same cadence, gear and wheel speed causing your avatar to slow down. Your bike computer doesnt know about this elevation change either so it just keeps reporting wheel speed as if nothing happened.

See this nice little writeup: How Does Zwift Determine My Speed?

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Hi Mark,

What do you mean it is random? There are elevation changes in Zwift, these aren’t random, they are part of the route. If you ride the same route tomorrow you will be able to compare time, speed, etc…

So my ride had 19m of elevation. Over 3km. Do these stats look right?

Sorry - wrong summary - this is my 3km 19m elevation.

Your speed doesn’t show anywhere on those screencaps, so we can’t tell if there were any random in-game speed changes.

I think the power, cadence, and HR data a looks good, though. You might have seen your speed decrease when your cadence dropped, though.

You think!? 139bpm for 69 watts? Unless Mark was intentionally riding in his lowest gear something feels off to me.

That being said, doing less than 100 watts up the little kicker at the back of the volcano.would definitely explain the slow speed reported. The question for me is why is the power so low.

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Averaging 69 watts is very low power, you may need to check that you’ve installed the bike on the trainer correctly.

Ok - so I’m not going mad - I have lousy power. But how could the trainer setup affect my stats? The speed is coming through at 24kph on the computer while distracted by trying to figure out the app. My average speed outdoors on flattish roads is 27kph for similar bpm and perceived effort.

Riding in near highest gear. And there aren’t any variables. If faulty sensors how would the mio505 computer have 24kph which is a more realistic speed?

I’m stuck…

Dumb trainers are estimating your power based on the power curve that Zwift has for your specific trainer and the speed of your wheel. They are not very accurate. Follow the manufacturer recommendations for how tight the roller is against your tire. Make sure the tire is inflated properly, it should be a slick tire too, not a knobby mountain bike tire. Lots of variables here… pay no attention to your bike computer

I have to admit that I didn’t really look too closely at the numbers themselves, just the pattern. 69 watts, though, does seem really low. Tire slipping on the trainer, perhaps? That might even explain the wild jumps in speed, if the tire is grabbing, then slipping, repeatedly.