I have no idea what you’re doing or expecting, you dont provide all the numbers and don’t even say if IAS or TAS is what you are talking about.
Maybe you picked some ice without knowing it? The model does not display it at the moment, you can check that in the Developer menu. Even if set to visual-only, it will affect your performance.
Just tried it out, following the page 13 in the ODM.
6000ft with OAT -20°C (manually dialed in), leaned to rich of peak, 2500rpm, full throttle.
Fuel Flow gauge shows exactly the 75% power, just above 12gph.
IAS gauge adjusted for altitude and temperature (6k at -20°C) shows about 132kt TAS.
If you check the diagram in ODM, I should get 133kt TAS.
I’d say it’s pretty spot on.
By the way: One more bug I found and reported:
MSFS in most cases doesn’t register the takeoff for the logbook. So it says 0 day/night takeoffs and 1 day/night landing. But departure airport, arrival airport and flighttime are correctly logged.
I just have 2 takeoffs in my logbook with the JF PA3 registered, but numerous landings… Not that I care, but some people want to have the ingame logbook correctly.