The PM is brand new and was installed by the bike shop when I bought the bike in June. I ride the Peloton almost every day this time of year, and it also has my FTP within a few watts of the Stages shows. I know the Peloton only averages power and is not a true PM, but the effort feels almost identical at all zones to riding outside.
I guess I could be stronger with my left leg and since it is a left side crank PM only, it could be 5-10 percent off.
The last time I did an FTP test outdoors I averaged 303 watts for 20 minutes on a flat course. My speed was 22.3.
This calculator may be helpful, seems reasonably legit. Works pretty well given my results on rolling-hill type courses (no megaclimbs) on a road bike (not aero-optimized TT bike)
Per that calc, 303w is roughly 22.9 mph so FTP test speed sounds about right
If your zone 2 is 160-200, on that calc:
160w is about 18mph
200w is about 19.7 mph
300w is 22.9 mph
So if assuming this calc is in the ballpark of right, 17mph for your z2 given your FTP seems quite slow, whereas your FTP test results seems only a a little slower than the calc.
Z2 is a big range - if you’re riding <160w, that’s likely too slow for z2.