I have a pair of Power2Max Type S meters: one on a Rotor 3D+ 175mm crankset with mid-compact 52/36 rings, and one on a FSA Gossamer 175mm crankset with a narrow-wide 44T 1X ring. I've been using them for about 5 weeks now. The power readings I get from both are quite consistent from ride to ride, and compare well to one another.

The problem I'm having is that the power readings seem consistently low. This morning I did 95 miles with 3,000ft of vertical @ 18.3mph, and the average power recorded was 179W. Simply plugging the numbers into the Kreuzotter calculator (height, weight, bike weight, elevation, etc) gives me a 208W average, and that's with no elevation figured. I genuinely don't think that a person of my size can manage that sort of average speed at 179W.

Another 38.7 mile ride with 2,400ft of vertical, 16.5mph, the calculator gives 198W, the Power2Max once again, 179W. Then 27.1 miles / 800ft, 18.5mph, calculator 215W, data from the meter 199W.

Another part of the problem, I'm comparing the recorded data from the Power2Max with what I used to get from my old Stages, which was consistently in close agreement with the calculator. For example, a 21.1 mile, 1,115ft ride @ 17.4mph-- Stages, 228W, calculator 222W.

I'm just guessing that the PM reading 15-30W low all the time isn't a big enough error for Power2Max to do anything about it mechanically-- even if it does indeed prove to be inaccurate. I'm hoping some other Power2Max owners can compare Strava data with the above linked calculator and see if their numbers jive any better.

The reason I'm concerned about it is that the primary purpose of my PMs is as "fuel gauges." I use the kJ (total work) to help me to better manage eating/pacing on rides. Now with the lower numbers-- today's longer ride, for instance, which only measured 3,350kJ by the PM measurement shows as 4,200kJ on the calculator-- it's messing me up over longer distances. I mean, 850kJ is a significant amount.

Found instructions in the Power2Max FAQ on how to determine if the unit calibration is correct:

"To minimize the amount of time you are, or might be, without a power meter we’ve developed a test that you can do to determine if your power meter’s power values are outside where we would expect them to be.

Here’s what you’ll need:

The weight of you and your bike (in kg)
A hill at least 1.5km long and 5-6% in grade
The actual length of the hill
The height difference (altitude gain)
And total time

I dug through the recent rides and found an effort of 1.9 miles @ 5% average, and plugged in the numbers. The PM on the bike gave me a 249W average, and the calculator linked by P2M (all in German, naturally) gave me 255W, which is within 2%. So I guess the Stages was just reading quite a bit high for... the whole time I had it.

Could be the Stages was reading OK but your L/R balance was not 50/50. Not that it's particularly accurate but what does the Power2Max say about your power balance?

I had to go look that up. Due thanks to the location of the arthritis, I have a pretty consistent 52L/48R split, which would account for a fair portion of the discrepancy between the Stages and the P2M, if the L/R split is to be believed.

I only have ~2,000 miles spread between the two meters, and put about 15,000 on the Stages, so I think a good bit more miles are in order to make any real conclusions. Regardless, I'm sure a new FTP test is beyond overdue... I just don't wanna do it.

I think you can also do a static torque test on the power2max to check that its slope is accurate. You’ll need a ~25kg accurately calibrated (i.e. not bathroom scale) weight which you hang off one of the pedals and then a Garmin to read the torque measured.