I have trained for a long time with a Stages power meter and a cycleops trainer, no issues there. As I started traveling a lot for work, I got the Garmin 3s and the feedback sports omnium trainer. The two set ups give me the same results, no need to adjust my FTP if I’m changing the power meter. I use each setup for 4 weeks at a time.
Last thanksgiving I got a Powertap P1S - stages pm is 175mm, cranks on one of the bikes I use for training is 172.5mm. The first workout with the Powertap and omnium trainer was really hard, I had to drop it halfway. It felt like the reading was lower but I thought I was just tired from traveling (I had just returned home). My next workout was a ramp test, I did it with the Powertap and got 256 W as my ftp, which was previously at 286 W. As you can imagine I got frustrated, installed the stages PM and did the ramp test again, same trainer, same fan, , same bike, same everything. The result was 283 W, within 1% of what I have with the Vector 3s.
Has anyone experienced “lower” measurements with the powertap P1s? I wouldn’t have a problem if the results were within a 2-3% range but 10% seems too much.
For the moment I set my ftp to 256W and will keep that while training with the P1s.
On a side note, I will consider the Vector and Stages readings to be correct as that leaves me around 3.9 W/kg 
I asked my friend to bring his P1’s around to test against my tacx bushido as I had a feeling it was reading low, however to my dissapointment the P1’s came out very similar but … I’ve just got a pair of garmin V3 and they read about 30W higher than my trainer. So perhaps P1’s do read low.
I’ve simultaneously run P1’s, a Powertap G3 hub and a Stages crank arm… The two Powertap units read within 1%, the Stages seemed to be about 3 or 4% higher than the two PowerTaps.
Given you mentioned different crank lengths are you sure they’re set right for all power meters? Could make a huge difference
Thanks for the replies, nice to know Powertaps can read lower.
I checked the crank lengths, the P1s is set by default as 172.5mm which is what I have on the bike I’m currently training. The Vector 3s is set at 175mm on the other bike I use to match the correct length.
I guess all this stuff is a nice excuse to get a smart trainer to see where my FTP lands using another measurement device haha
For a variety of reasons I’ve ended up with a number of power meters. All crank/pedal based one sided ones. Fortunately 3 of them have the ability to input a correction factor in. Taking my Stages as a base as you can’t on that I’ve put the following corrections in to get them within a couple of % of the Stages.
4iiii - +5%
Favero Assioma Uno Pedals +8%
Zwatt Zimanox - +20ish% (unfortunately the limit on this one is +10%)
Not that you would need to but did you happen to check how close Virtual Power was in relation to your power meters. I’m about to upgrade from VP to a Powertap G3 Hub and am curious how much of a power difference I can expect.
I started using TrainerRoad on Virtual Power, about 2 years ago. I got an FTP bump of around 20% when I got a power meter. I was using a second hand CycleOps Fluid2 trainer that spent a long time not being used in someone’s garage, this can explain why virtual power wasn’t very accurate.
I noticed my feedback sports omnium has lower resistance on high power (VO2 workouts) as I have to shift to higher gears to get the same power output as on the CycleOps.
In short, I guess the power difference you can expect is highly dependent on how well your trainer follows the power curve provided by the manufacturer…
Consistency is arguably the most important thing. If you change power source, retest and use that figure for the powermeter you tested on.
Don’t get hung up on getting a power boost or drop if you change powermeter. Your body is still producing the same, it’s just measured differently.
FWIW, depending on the trainer resistance setting I used when I was on virtual power, I could have an FTP of nearly 500 “Watts”.
Thanks
I’ve the same fluid 2 so It will be interesting to see.
Some of the inaccuracy I believe comes from the oil heating up as the time and effort increases
I agree, stick to the same setup, follow the TR plan and check your results. Unfortunately that is not an option for me as I have to travel every month for work.
Using more than one power meter makes things harder, it’s like my buddy likes to say, “a man with two watches cannot tell the right time” haha.
My friend just bought some Vector 3’s for outdoors. He already uses a Wattbike for Trainerroad so he wanted to compare them and for a 30 minute recovery ride (can’t remember which one) the Vectors came out about 17 watts (8%) lower than the Wattbike. I loaned him my P1’s to try and they also came out virtually the same as the Vectors on the same workout/time. Both Pedal based PM we dual sided.
He is thinking now the Wattbike may be out and is going to get another friend to compare his Wattbike.
Sorry for the late reply - I’ve never used Virtual Power so can’t compare I’m afraid…
I couldn’t reconcile power between multiple Wattbikes and my Vector 3s pedal. I thought initially that my Vector was out but since realised it was the Vector that was more accurate.
I know old posts… but a crank/pedal based power meter should read a little bit higher than a hub based power meter due to transmission losses. I’m wondering even the best chainline doesn’t lose some power from the crank. But may be a topic for another thread.
As above, make sure any pedal based power meter is set to the crank length of the bike being used.
2 years ago one of my pedals (dual) decided to default to a crank length 5mm longer on one side. It ended up unreading massively and confused the hell out of me for 3 days until I figured it out, thinking it was something to do with me physically 
Take a look at DC Rainmaker’s blog post on trouble shooting power meter problems: How to: Troubleshooting Power Meter and Trainer Accuracy Issues | DC Rainmaker
FYI, My Assiomas match my Neo, my old stages pm was bit all over the place.