Two powermeters with different methods of measurement are unlikely to report identical instantaneous readings, even if they are identical when averaged. Here's a few reasons two powermeters can disagree on average:
- Measuring power at the crank (4iiii Powermeter) vs measuring at the wheel (Fliiiight): Both readings are correct for the thing they are measuring, but a crank mounted powermeter measures your power before small losses due to inefficiency in the drive train, and a trainer measures it after. This difference in measurement should result in very small differences in average power, in a consistent direction.
- If your left/right power balance changes throughout your ride as you become fatigued, the measurements from a single-sided powermeter can diverge slightly from the trainer, which measures the average of both legs.
- One of the powermeters may be miscalibrated.
Fliiiight has highly accurate power measurement (+/- 1%), however, if you wish you can calibrate Fliiiight to match another powermeter, for consistency between your indoor and outdoor rides.