Major differences in power indoors vs outdoors and possible workarounds

Good day everyone. I having issues with my trainer power compared to outdoor power. Here is a description of the issue:
I have a tacx flux 2 trainer. multiple ftp tests put my ftp on the indoor trainer at ±200w. On my mtb which I only use outdoor, my last ftp test put my ftp at 265w. I train every week both inside and outside. I currently use training peaks to schedule my training. on training peaks, i’ve set my ftp to the outside value (265w) and build my workouts using percentage of ftp rather than a fixed power number. On zwift (app I ccurrently use for indoor training) i’ve set my ftp to the 200w value. The percentages automatically set my power to the correct values for indoor training. when I sync my indoor workout back to training peaks after I completed it, I just manually change the TSS on training peaks to what zwift showed I have done. This will keep my CTL, ATL and TSB correct and accurate. Time in zones however aren’t accurate. Now I was wondering, is there a way to manually calibrate my trainer to read roughly the same power as on my outdoor rides? even though it will mean I artificially inflate the power numbers on the trainer, at least my power numbers will be similar and I can use the same ftp on both. I mean, for example if I do a set of tempo intervals outdoors, it will be at around 225-230w but indoors it will be at 160-170w. The power differs but the adaptations should be the same, so if I can get the trainer to read higher, match the power on the trainer to what I do outdoor and use the same ftp value, in theory I should be able to use training peaks’ time in zone feature and have it more accurately? I can also then use trainer road for the ai machine learning to build programs for me but only if I can change my trainer’s output readings. Otherwise the data on trainer road will be so different from outdoor rides to indoor rides that the ai features will be useless. Or is there a way to use two separate ftp’s on TrainerRoad for indoor work and outdoor work?
There are differences is my two setups which could be the reason for the different power readings.
My indoor setup: Road bike that is a size too big for me. Round chainrings and trainer measures power after all the drivetrain losses. I don’t have a power meter on the road bike. I only use my road bike on the trainer since I got the trainer.
My outdoor setup: MTB that is more my size. Oval chainring. Spider based power meter that measures before all drivetrain losses. The power meter is a Xcadey.
Note: I have optimized my cooling for indoor training, I focus a lot on hydration and fueling both for indoor and outdoor work. Heart rate during indoor and outdoor work suggests the effort level is very similar even though the power is so much lower.

Any suggestions?
Thanks

Put your MTB on the trainer so that you can use the same power and fit both indoors and outdoors?

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I’m not sure that it’s possible to manually change the calibration on your trainer, that would be particularly troublesome for people that do virtual races since it would show a much higher power output than the user is actually putting out.

I would dig more to find out where and why there are these discrepancies exist. I think such a vast difference in FTP is due to some sort of equipment or calibration issue as opposed to fit/bike differences between the two items. Plus, the geometry of the road bike is probably more suited for longer term power vs the mountain bike.

My Wahoo V6 trainer, Garmin power pedals, and SRAM red crankset power meter are all within around 2-3%.

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I should probably test the power with my MTB on the trainer, then I can also compare power from my trainer to the power meter on my MTB. I always though the lower power indoors just came down to not being able to produce as much power. Reviews of my trainer shows it isn’t far off from the Favero Assioma power pedals. Also, the Xcadey PM om my MTB is also close to the Favero Assioma, even reading slightly lower.

I don’t want to regularly use my MTB on the trainer for actual training because I like to have my road bike permanently set up on the trainer since I only use my MTB for outside rides and never use my road bike outside anymore. The permanent trainer set up makes it so much easier and quicker to get in some training, that’s the main reason why I use my two setups the way I do.

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