Speed issue on hills


Not sure if this has been an ongoing issue for me or new, but just noticed it two rides ago and it was there again today.

I have a Kickr and Garmin head unit / Zwift running on Mac Air (10.10.4).  

When I’m going uphill, Garmin will show numbers perhaps double what Zwift shows.  Going downhill it’s the opposite, Garmin will show much slower numbers than Zwift.  

Not a big deal, I’m limited to short infrequent rides - recovery from multiple surgeries - but wanted to see if I had jacked with something and not realized it.  

Thanks in advance,


Is your garmin reporting speed from a speed sensor on your bike?  Zwift simulates a virtual speed based on your power output (power meter or calculated power) so this means it will, for a lack of a better word, slow you down uphill and speed you up downhill to compensate.

I believe Garmin is getting speed off Kickr.  

I wondered if there wasn’t some math getting done somewhere as it just seemed too predictable. 

Not that I’m in front of app / trainer, but just hit me, there’s no “speed” sensor right?  Trainer or power is upper left and on the right is cadence and there’s one for H/R and trainer is there somewhere, so again, guess this shows speed is getting calculated rather than pulled from a device.  Interesting…

Thanks for the insight!


While speed sensors measure speed and distance based on your physical tire rotations, Zwift measures these based on watts, course incline, your weight, and other factors - such as whether you’re drafting. Because of these different factors, it’s not unusual for your speed and distance in Zwift to differ from what you’re seeing on other equipment. The two will rarely, if ever, match up.

This is what  the “trainer difficulty” slider in the settings screen controls. If you slide it all the way to the right, Zwift will apply 100% of the gradient to the KICKR. The default is 50%. You may still see differences in speed, but they should be smaller.

I am new and a couple months have gone by since the Aug post noting the slider in the settings section. I do not have that choice in my settings, has it been removed? If so what percent is being applied?