Hi,
I have installed a 2023 Stages 4iii on my bike for the outdoor season, however I have significant differences between the Hubs and the Stages wattages, see the attached picture:
On the left side you see my Stages 4iii captured via my Fenix6 and on the right side you see the Zwift Hubs recorded data.
As you can see, the difference is great, between 5 and 15 percent, which ends up in a claimed wattage of 250 watts for the 4iii against 225 for the zwift.
Yes, all the internet is full of this.
Bottom line: Stick with the values from one device for training and don’t worry too much. There is no tools in this market, just expensive toys. I went for a replacement of my 4iiis power meter through the retailer and the new unit did track my smart trainer better. After a replacement of the smart trainer, I’m back to a difference.
The 4iiis offers adding a scale factor, so this could help you get values that are comparable. I usually do FTP tests for indoor and outdoor season and always stick to the numbers for one specific device.
There are a lot of reasons for differences like this, and the number one thing to know with data comparison is that it is a rabbit hole. For most people these differences don’t matter so unless you really care about your data or at a level where it is expected to be able to proof your performance, dont spend too much time on it. Pick the one that makes most sense and go from there.
If you want to do more it’s important to know if it’s a 4iiii or a Stages and if it’s single or dual sided. These are seperate companies and 4iiii in general has a better track history of providing accurate powermeters especially if your are running 11 speed shimano. If its single sided there is a chance is measuring the leg thats providing the most power for you therefore skewing the numbers upwards.
If its possible check that the correct crank length is set in the powermeters app. For a lot of powermeters the wrong length will get completely wrong numbers.
For the Hub its important that its calibrated properly or running autocalibration, for the most correct numbers. Also important that your drivetrain its relatively clean and lubed, as poorly maintained drivetrain easily can have around a 10 watts loss.
If all of this okay, then one or both of you Powermeter/trainer are definitely not correct and should be replaced under warranty. The only way to get a better guess at which one is incorrect is compairing it to a third trusted source. Might be you know someone who has a pair of Assioma pedals or a Kickr trainer you can borrow.
This is still only a guess as you can’t be completely sure without testing against many sources, but it will give pretty clear picture if its align closely with the trainer or the power meter. Again this is a massive rabbithole, so unless you need to do it, it can be a lot of wasted time and energy for something that doesn’t really matter
