Your recorded numbers will be too low. Same force, shorter crank = lower torque = lower recorded power.

It’s a linear relationship, so your readings are 1.4% too low.

If it matters (e.g. you were doing an FTP test), you can adjust the .fit file on fitfiletools.com

Otherwise, I probably wouldn’t bother.

[Note: I misread the original post and got this back to front, so have edited to correct!]

Makes sense. Ta.

Is it the other way around... if his crank was set to 172.5, but the actual length was 175, wouldn't the power calculated and displayed by the P1 be 1.4% low?

You’re quite right. I misread the original post. Apologies all!

I’ve edited my reply to correct.

That's the way I calculate it as well. P=Tw and T = F*l

P = F*l*w

So the same force and cadence on a 175 will result in a higher power than the same force and cadence on a 172.5. So by entering in a shorter crank length in the formula, you're recorded power will be lower.