Power Meter Numbers

I would like to start by saying TR is an excellent training partner and the advice given on the podcast, is top notch!!
My question relates to power meter data, and how it is different on my new bike. On 7/27, I was hit by a pickup truck while riding my bike. Thankfully I have very minor injuries. My old bike was Diverge and I was using a Stages non-drive power crank. It was also a 2x11. My new bike, another Diverge is a 1x12 and I am using the Garmin Rally single-sided power pedal. I have noticed that my average power is much lower now after completing similar rides with similar effort. Is it because of the 1x setup and the 40t chainring?

Sorry to hear about the crash but sounds like you got away with minimal issues.

Despite what we would like to be the case, power meters are not guaranteed to “match”.

  • In your case, you introduced a new power meter and it’s possible for them to give different data even if they both fall within their defined power data tolerance level (+/-1% for those from memory?).

  • Then we have to consider the many examples of people reporting power data discrepancies between meters that exceed any/all stated tolerances. The reasons for these vary, but large splits are not uncommon.

  • Have you properly installed and calibrated (zero offset) each meter as defined by each maker? We need to validate if these important steps are covered, because they are critical and sometimes overlooked. Garmin pedals in particular have been seen to give some questionable data when not torqued to the proper setting.

Per the many issues above (and likely more I didn’t cover), it is not appropriate to compare data from one meter to another unless you have actually compared them simultaneously. Because of that, the most common recommendation when swapping power data devices is to perform a new FTP test. It’s possible that you would get different values even if the only thing you changed was the power meter.


I’m glad it was only minor! You’ll find lots of post on power meter numbers but basically you can’t compare any of them directly even in the same brand. Where they are placed (crank/pedal/hub), dual v single sided, +/- errors are just some of the reasons for differences, All that really matters is that the power meter you are using is of a good enough quality to be consistent from one ride to the next.


Over the last 3 years I have had a stages, a 4iii, a power2max, and a set of rallys (I have the most time on these). Overall the stages and 4iii read quite a bit higher than the rallys and the power2max, about 5-10%. My feeling is that the stages tends to read high, and the rallys are quite a bit more accurate.

I strongly, strongly recommend just re-testing everytime you get a new PM. When I switched over to the rallys and tried to use my old power numbers, they were super out of whack and I stubbornly kept trying to hit unrealistic numbers. It was really demoralizing, and also absolutely buried me.


Agree with all the above.

You need to have both PM’s at the same time to be able to set them up the same/compare

As it is, you just have to use your new numbers as the “,truth”. That may mean setting new FTP, creating new seasons on TR, and resetting your expectations.

If you get another PM you can compare the two and know the difference, or use an offset. Without having them both at the same time it’s impossible to know exactly where it lies


Despite what others here have said, you don’t need to have two power meters simultaneously in order to know how they compare – but you do need to be careful with the comparison protocol. If you had done a careful “baseline” test with your previous PM you could repeat that with the new PM. That’s what I did when my previous bike and PM were stolen – I’d had a careful set of tests so when I got a replacement bike and PM I was able to compare them.

Can you give an outline on the baseline & test protocols you’d recommend?

I came up with VE as a consequence of doing PM testing: originally I was looking for a way to compare the quality of power data against a known benchmark (the same road profile, whether it was flat, rolling, hilly or a combination). Then I realized that good speed and power data could be used to spot dips in the road of a few centimeters. That’s when I realized that this analytical technique was pretty sensitive and replicable. So testing power meters for data quality is just the reverse of CdA/Crr testing.

The best baseline test is to ride on a smart trainer with the bike that has the other power meter. The difference between the on bike power meter and the smart trainer is what you’re looking for.

5-10% is normal. I had 3 different stages left arms and they all read within 5-10% different. Yes, it was a sad day when my effective ftp lowered bc my unit died and I got a new one that read lower. Such is life. Also, my stages reads lower than my cycleops hammer smart trainer, tho one is a left side only and the other reads different.

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I tested stages carbon left crank, p2m and s-works double PM against trainer and all three were in 1.5%

Depends on the kind of drive train and how well maintained they are there can be a few watts in drivetrain losses too.

I have Garmin rallyxc on my gravel bike and assiomas on the road bike and the Garmin read somewhere in the region of 6-8% lower. Just accept the new value as normal the watts you have are only important to compare to yourself. ( although if you are racing Zwift take the PM that reads high…).

I have not done any real-world comparison other than looking at data from my old setup to my new one. I do calibrate before every ride. I used the AI FTP detector and it did increase a bit since I installed the Garmin Rally pedals. If I were to perform a normal FTP test, it would be on the trainer and using the power meter built-in, and not sure it would show any real data for the Rallys.
I could install the pedals on my old Diverge that is now dedicated to the trainer and use that for the FTP test if I am really concerned.
Thanks everyone for the responses. Cheers.

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I’ve compared my 4iii’s to my turbo, and they were close enough for me (within stated tolerances). With AI FTP and AT, does it really matter as much? Surveys, AT and AI FTP will take care of any differences?

If you switch between PM a lot for example outside bike / trainer bike or road/gravel it is important to know the approximate difference else you go to hard or easy at least if there is a 5+% difference