Fuel settings?  Perhaps you are only running on one fuel tank, so the balance is out?  Large airliners rarely even notice this, as they outweigh the gas by enough.  But a small single or twin prop is going to notice big time, especially with full realism.  Maybe that?  Or perhaps the fuel loads got messed with somehow.
From my stock Mooney:
LeftMain  = -4.1, -7.9, -0.8, 47.5, 3.0       
RightMain = -4.1,  7.9, -0.8, 47.5, 3.0       
fuel_type = 1                                 
number_of_tank_selectors = 1                    
electric_pump=1
And you might try redownloading the Piper to see if there is something awry in there.
But then again, it could be a gauge problem . . . that would be a real bugger.  If its that, and the Mooney is affected, that means a stock gauge set got replaced.  You'll have to get into the disks to find it again.