My FTP outdoors is about 5% give or take higher than indoors. I’m using the same bike a power meter (Garmin pedals) both indoor and out. Why the difference? TR seems to think there should be none, but there are other “experts” out there that acknowledge the difference. Is there a bottom line, or is this another “let a thousand opinions bloom” and we’ve no real idea which one is correct?
I am not sure if you ride a wheel on or a wheel off trainer for indoors or even rollers. it is a known fact that riding a trainer indoors, your pedaling dynamic changes slightly due to the nature of the trainer itself. It all has to to do with the differences between the flywheel resistance and rolling resistance and and how the load is transfers back into the pedals. Once you transfer to outside riding it takes a short time for the dynamic to change back. That slight difference may be causing a slight difference in your power delivery. who knows if its 5% different or not. To say there is no difference is between the indoors and outdoors is presuming the differences are just negligible IMO
I forget where I saw this, maybe knowledgeiswatt social media, but I believe there is science to support the idea that your outside FTP is generally 3-5% higher than indoors. Things like cooling via wind, micro-position changes, etc can contribute
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Welcome to the TR community!
Your indoor and outdoor FTP should be the same. The biggest difference when training indoors often boils down to cooling.
When you’re outside, you have a lot of air moving around you, while inside, you’re standing still. Without proper cooling, it’s very difficult to put out the same power as one can outside. It even becomes impossible to do so in higher power zones because the body overheats and can’t keep up with the work demanded of it.
To mitigate this, we recommend investing in a good cooling setup if you haven’t already. We really like industrial strength fans or blower fans that can move a lot of air. I’ve personally found that one blower fan does the trick for me, but other athletes even have two or more of them if they struggle with staying cool inside.
Beyond that, there are other aspects of training indoors that could affect how one might express their FTP. The mental aspect is probably the next biggest reason behind cooling. I like listening to music and watching races during hard training sessions or watching TV shows/movies during longer Endurance rides. Other athletes like using platforms such as Zwift. I heard a legend that Eddy Merckx and the old-school racers would stare at the wall for hours while on the rollers to build mental fortitude… You’ll have to find out what works best for your own mind. 
As other athletes mentioned above, there are also differences in terms of bike positioning and how pedaling feels when you’re on the trainer compared to outdoors. In general, though, those differences are pretty small and most athletes get used to them quickly. If you feel any pain or extreme discomfort, it might be worth seeing a bike fitter to get that checked out.
If you can stay on top of cooling and the mental side of training indoors, though, you should be able to get your indoor and outdoor FTPs to be the same. 
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More cooling (better airflow), different muscle engagement, and sometimes just the mental boost of being outside all play a role.
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I have a very similar experience. I feel like I have quite a bit more power outdoors than on the trainer. I have a low of wind (two wahoo fans and two lasko fans) and I think I have 5-10% more power outside. I was worried about it enough I took my outside bike and put it on my trainer and recorded two power files to see if there was a difference and they were both spot on.
I came here for advice on how to handle setting a 1hr power PR outside that was higher than my FTP. I got a range of advice from let ai ftp do its thing to how could you not increase it based on what you have done. I decided to leave it and follow the ai recommendations mainly because I am more worried about consistently getting on the bike and I don’t want to bump it up enough that I fail workouts.
Hope that perspective helps.
Mine is 10-15 watts higher outside. Or at least that is how I feel it
Same power on trainer and outside here. But motivation can be different at times making it seem different. And +1 on cooling being key. I’ve got a mini split in the garage that makes it cool/dry (66f) and then 3 serious fans (lasko style being the weakest). If sweat is ever hitting the floor before it evaporates on your body, you don’t have enough air moving or the air is too humid. During Texas summers, indoors is a better option for intervals because quality cooling outside becomes impossible (even at 5am).
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Mine is the same indoors and out 
. Just need proper cooling which is more than you would think.