I recently picked up a new bike and a set of Garmin Rally RS 100 pedals to help aid in my quest to Challenge for the Podium in 2023,
After riding all winter indoors with my Tacx Neo2T, I thought it would be great to compare the power variance between my Tacx and the new Garmin pedals.
As I prepared for the test, I anticipated some level of variance to occur for a wide variety of reasons, including reported tolerances, power meter placement, single vs dual-sided, and I’m sure many more factors I don’t even know about.
After over seven hours of riding across six different sessions, I am surprised at just how large the variance ended up being, especially when riding at lower power levels where the Tacx was measuring a consistent variance of 12% lower than the pedals.
Read on to learn how I set up the test, the different factors that can create variances, the results by power band, what feedback Garmin had to provide, an additional comparison vs my older Stages Power Meter, and final thoughts.
Garmin Rally RS100 vs Tacx Neo2T – Setting up the Test
As I prepared for this test, I did my best to ensure that everything was tracked as fairly as possible, including:
👷🏻Calibration – Ensuring both trainers are calibrated before each ride (not required for the Tacx), have the correct firmware installed, are installed to specifications, and have remained in a room with a consistent temperature.
🧽Clean, Oiled Drivetrain – For the test, the Garmin Rally RS100s are installed on a new bike with a new drivetrain, with the chain on the largest chain ring and the middle cog in the back.
💪🏻Workout Mode – During workouts, the Tacx Neo2T was set up in Zwift using ERG mode, with Zwift leveraging the Tacx Neo2T for both power and adjusting resistance.
🛣️Freeride Mode – I also ran two long ride sessions in Fulgaz with Fulgaz tracking power from Tacx.
💻Garmin Setup – For both the workout and freeride sessions, the Garmin Rally pedals were connected to my Edge 530 bike computer for tracking power and cadence for all rides.
🚮Settling In – The first ride after installing the pedals and the first ride after retorquing the pedals did not have their results included, just in case the pedals needed time to settle in after some hard forward cranking efforts.
Factors that Create Power Meter Variances
A variety of factors can come into play when comparing different bike power meters, including power meter accuracy, drive train power loss, single and dual-sided power splits, and I’m sure many more.
Both the Tacx and the Garmin pedals have a claimed accuracy of 1%. Depending on the way that variance swings with each device, we could end up with a 2% gap due to manufacturing tolerances.
The Garmin pedals are installed on the crank, with the Tacx measuring power from the hub. I read a great article by RideFar that goes into detail about drivetrain losses. A few interesting stats, most cycling models assume 3% total drivetrain loss. They suggest 5% is a more accurate number for the average rider, with an old chain adding an extra 1% of power loss.
When riding on my Tacx trainer, power is measured as a combination of both my left and right sides. I own the Garmin Rally RS100, a left-side power meter, meaning that any discrepancies I have between my legs in power output could muddy the waters of this test.
Garmin Rally RS100 vx Tacx Neo2T Variance Test Results
Now that the test was set up, it was time to ride. Let’s see what that looked like.
Here’s a picture from a typical training ride, courtesy of Zwift Power’s handy analysis feature:
This file is remarkably similar to each of the six other rides I did. A few things to remember, Zwift was using the Tacx Neo2T to track and manage power, so it is providing an accurate reflection of what the app was targeting.
Where above, we have what a typical ride looked like. Via the chart below, I’ve broken out the totals of all the rides combined, segmented into different average power targets.
As you can plainly see, the difference at lower wattages is huge, with a 135-watt recovery interval on my Tacx measuring in at 151 watts on the Garmin Pedals.
It’s also fascinating how the variances only seem to occur at lower wattages and how once the average watts hit 180, any differences can easily be explained away by anticipated losses through drive train inefficiencies.
Stages vs Tacx Neo2T Power Meter Comparsion
Due to some ongoing renovations, it took me a little longer to get this post out than I originally anticipated. I had hoped to add a few more pics, but those will have to wait until I can access my desktop again. Although I wasn’t able to get the pics, I was able to hook my road bike and Stages Power meter until the Tacx for a few rides.
I will call out that I did not put the same rigour into preparing for this test as I did the previous one. That said, the Stages power meter was calibrated before each ride, the drivetrain was recently cleaned, the bike has remained in a consistent environment, and the general setup was the same.
Analyzing the results, the surprising thing for me was how remarkably similar they were compared to the Rally pedals, with similar variances at both the low and high end.
Connecting with Garmin
I reached out to Garmin, and they quickly validated that my findings were not unexpected, noting…
“with how the trainer calculates power it’s not uncommon to be less accurate at lower wattage” and “the pedals should be more consistent”Garmin Support Live Chat
Support also shared two useful articles. The first, My Power Meter Output is Different From My Smart Trainer, details the many steps that should be taken before doing a test like I did. The second was an email sharing their instructions for how to conduct this type of test using the Tacx app. I have not yet conducted this specific test, but I can confirm that my methodology aligns nicely with my own testing, with the main difference being that they specifically call out having your gearing set to 50/23 or 34/19, using a steady 80-85rpm.
The main takeaway I’ve learned through this experiment is to pick one power source and always use it for all your training, both indoors and out.
By picking one power source for all your rides, you will be able to ensure that your indoor training efforts are 100% aligned with outdoor training and race targets.
More importantly, if you are a rider whose power target is below that 180 mark, you will need to be extremely cognizant of the risk of using different power meters on different rides and the impact that the switch can have when moving off the trainer.
I hope you found this article as interesting to read as I found the experimentation process.
I’d also love to hear your comments on moving between power meters and any specific things you’ve noticed or adaptions you make when moving between different setups.
Thanks for reading,
Hey! I enjoy reading your blog.
Good for you for pulling a test together. I often wondered what the variance is between my TacxNeo and outdoor bike power meters. I have a few different bikes/PMs and – garmin rally pedals on my gravel bike. I definitely notice that garmin is more lucrative with its power readings at cruising altitudes compared to the Tacx 🙂
I try not to think too much about it – I know that coming inside after a summer of outdoor riding really hurts on that Tacx and is an eye opener (and sometimes makes me cry haha!) Hoping it makes me stronger too – so it’s worth the turmoil.
Thanks for the kind words. The inspriration for this post was that my first trainer was a Tacx Genius and it’s power readings were crazy high. When I picked up my Stages Power Meter my FTP suddenly dropped from 265 to 220ish, making me forever curious about that variance when moving between sources, especially as I look to start basing my outdoor race paces on a new system.
I try to stay with one main PM while outside (eg. my TT bike) and do an FTP ride with it. That then determines my pwr ranges for races…very helpful. Once back indoors, same on the Tacx (FTP then work that through the winter months). Lucky for me, the pwr readings are within an acceptable tolerance between those two devices. Not that I’ve done a test like this 🙂
I am interested in how you’ve set up your running power. Do you have a Garmin watch? Since I upgraded the software on my Garmin 945, stryd power is very messy – the data doesn’t flow through nicely to training peaks nor is it easy to follow run workouts on the watch. Do you have the same issues? If yes, how did you solve them?
DM me on Instagram and let’s figure out a time to connect. I have it setup on my Garmin 945 and it seems to be working more accurately since I tweaked the settings. We can go through and see how our settings compare. That said, I don’t currently use it for workouts, as I find I just prefer running off pace/hr. I’m considering trying for a month, but I’m not there yet.