I have a Stages crank arm power meter on my old bike. I just bought a Specialized Tarmac Pro, that has a 4iiii crank arm power meter. The power difference is substantially lower, like 15-20%.
The shop put on a new 4iiii power meter. Everything has been calibrated several times with Firmware updates. And there has been no change in my power numbers.
Has anyone else had any issue like this?
Can the Stages still be calibrated?
What are the other specs? Crank arms, gearing, etc?
Just to be clear
The shop replaced the 4iiii with a new 4iiii and you are getting the same readings from both?
That is a different reading from your old stages on your old bike?
Crank arm length is the same.
Yes, the Stages pm has been calibrated.
Gearing is exactly the same.
Pastor Gary Isbell
Yes, I had the original 4iiii power meter replaced with a new one.
I use my Stages power meter on my Cannondale Super Six.
How is the battery in your Stages?
If it’s a new battery I’d say the Stages is reading low. Sorry.
Are both one-sided power meters?
Literally everyone who has used more than one power meter! 
Unfortunately I think the only way to be sure it to add a third measuring device and see which one that agrees with.
Do you know anyone with a smart trainer you can fit your bike to?
Some have said that the Stages are sensitive to batteries. And watch for those new coated batteries, that coating can play havoc with the electronics. Use an abrasive wet ‘shop towel’ to try to remove that so it makes better contact. There are some that turn whatever they touch blue too apparently to make kids want to eat them more. 
But coin batteries are no joke: They can kill small kids and pets! Keep control of them and make sure they are disposed of properly!!
Confucius say “Man with one power meter knows exactly how much power he is putting out, Man with two hasn’t a bloody clue”
Check the bolt torque of the PM on both bikes. The toque needed on a Shimano crankarm is about 2x what you think it should be. If you don’t have it absolutely clamped down, you’ll look like a HERO.
Otherwise, position on the bike will make a huge difference. I put a 4iiiiiiiiiiiiiiiiii on my MTB and the numbers were 75% of my road bikes. I bought some PM pedals thinking it was full of it. Turns out that I just can’t make 20min power on my MTB for some reason.
My main power meter is my Stages. It would read up to 75 watts lower than my old piece of crap Vortex trainer. My v2 Kicker reads 20 watts low across the board. My old Powertap hub was within 5 watts of the Stages.
Your best bet may be getting another 4iiii.
I always thought my single sided assioma over estimated my power when compared to my flux s trainer. Unfortunately that meant a 20-30watt difference in my inside and outside power.
I now have some powrlink pedals the flux s and powrlink are pretty much within a couple of watts of each other and consistency is what you need.
Just use the data from the power meter you’re currently using as that’s the only consistent data point.
Yes.
Thank you.
Batteries in both devices are brand new.
Thank you
Bahaha
I’ve got the Cannondale and Stages power meter bike on a Wahoo Kickr now. Numbers are still off…
Thank you