It won't matter on the trainer as long as you always use the same power measurement device.

However, outside (races) you will only have the pedals, so race target power has to based on their test results.

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"...the street finds its own uses for things"

Obviously the higher one is correct.

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http://www.sfuelsgolonger.com

If you are using power match... there really shouldn't be a 10 watt difference. TR should report what the P1's are doing.

The Garmin might be 1 or 2 watts different... but not more.

I don't see why you have that big of a discrepancy. I use an SRM + PM and I am never more than 1 watt off between what my Garmin reports and what TR says. The only reason I even use the Garmin is for live info about "lap" info.

You're running HR is likely a good chunk higher than you're biking HR. You're better off doing a running test and knowing for sure.

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--Chris

The reading differences you are seeing is 4.5%.

Most power meters are accurate to 1-2%, so that could be some of the difference. I think it would be a stretch to say that your P1s were running 2% high and your kicker was running 2% low to make a 4% difference, but a straight 1.5 to 2% difference would be plausible.

Now factor in that your reading power in two different places that are at opposite ends of the drive train. That could be another 1-3% depending on how smoothly your drive train is running. So it could be explained.

You mentioned that you did a spin down on the trainer, but didn't mention if you calibrated the P1 pedals. I don't own them, so I'm not even sure if it's necessary.

In any case, unless you plan to execute and FTP based pacing plan only on the trainer and never outdoors, I would go with the FTP from the pedals, assuming they were properly calibrated.

How are you transmitting to trainer road? Bluetooth? TR only takes the left leg and doubles it if using bluetooth, and I believe with a dongle it will give an accurate split.

I ran into similar confusion a year ago with P1s and TR reading about 10 watts higher than my edge. Went through multiple tickets with Garmin, powertap, and TR before stumbling onto that answer somewhere else (really don't remember where). Try it out though, do single leg work with the right leg and TR will give a reading of 0 power, but if you do left leg only the power will stay the same as if both legs were pedaling.