i have a power2max and was comparing it with my old powertap (pro). Difference is about 40 watts in favour of the powertap on my training course (speed/cadans/heart rate is nearly the same). Tried it a few times now and it is always between 30 and 40 watts.
Any ideas?

If it's always 40 watts it sounds like the intercept is wrong. Have you tried stopping pedalling for 2secs so it re-calibrates? How about calibrating from your garmin? Could also be the powertap that's off. I have both and ran them together once with a 5-10w difference during a TT.

thanks, will try that

i have both as well and ran them together on different intensity-levels: very close together, difference most of the time between 4-8 w. at around 200w it was a little bit more, ca 6-10w.

in the worst case, you have to send one of your systems to the manufactorer who has to recalibrate it.

there was a case similar like yours, concerning p2max, in german "tour-forum. the users powermeter showed to low numbers. he got it recalibrated by "saxonar", and it worked fine.

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sorry vor mai bed englisch!