Watts estimate a about half using garmin speed/cadence

hello all,

i have my old bike on my trainer so no PowerTap.  I’m using the Garmin speed/cadence sensor and cycle ops HR sensor.

the watts I’m getting in Zwift are about half what I’m really putting out. 

Any thoughts on how I might get that to be more accurate?

thanks, and Happy Holidays!

How do you know what watts you are putting out if you are just using a speed and cadence sensor? 

What size wheel do you have on your bike?

what trainer are you using?

is the trainer supported within Zwift?

need more info to assist you.

Hi Paul,

Here’s the details:

I know it’s off because of heart rate.  

Heart Rate of 160 = 145 watts in Zwift’s estimate.

I rode my other bike with my Garmin and PowerTap the other day.

Heart Rate of 120 = 180 watts.

Big difference.

My trainer is old. It’s a first gen CycleOps Fluid.  So no, not supported.  I did play around with tension on the wheel, but it didn’t seem to make much difference.

Wheels are 700s.

Maybe it’s just the trainer?  Is there any workaround?  I suppose I could play with my weight in Zwift.  That might give me more power?



Your estimates based on HR could be way off. You have different variables while using an indoor trainer. How long did you do the test to compare the indoor and out door sessions?

Unless you use the same bike with the power meter there is no way to tell if the watts are off.

I would suggest putting the bike with the PowerTap onto the trainer and see what the watts are. Just remember that it is different riding on an indoor trainer compared to riding out doors on a road. 

Changing your weight will not give you more watts, but it will give you a higher watts per kilogram. 

I was on a different indoor trainer looking at the HR and watts, but point taken.  I’ll bring that bike home next week and give it a real test.

Thanks and Happy New Year!