Inside Power v Outside

Hi….been a TR user for several years and have been trainer riding for 25 + years. Recently bought a Wahoo Kickr and love it. Started riding again outside last week and rode well…. actually led a good portion of the way. After the ride my buds complimented on my riding and asked what my FTP was and I said 136. They laughed and said that’s impossible! They said “ No way it’s that low and you rode the way you did today”. I don’t have a power meter on my bike. Only thing I have to go by is HR and speed. I was 18 mph and HR was what I would expect for sweet spot efforts. I pulled the group for a couple of longish pulls 15-20 min. The effort seemed equivalent in intensity/effort as a 12-15 minute sweet spot interval on TrainerRoad (FTP 136). I use erg mode on Kickr…cooling system hydration etc….I am 61 Wt 131 bike wt 16.6. I had this problem last year also but attributed it to a tire on old KK rock and roll trainer. I no a number is a number and as long as my TR #’s improve I am improving but it is annoying. Does anyone else experience this. Input would be helpful. Thanks

Generally accepted that Outside is higher due to many factors, cooling, motivation by other riders, can also be a combination of old FTP test data indoors, trainer calibration ( trainer is 1% low and PM ( if you had one) is 1% high, still in range for acceptable but 2% variance if everything zeroed out. ) so all these add up to what you are seeing as a difference in performance.
I look at indoors as “training” and outdoor rides as “testing” but with the outdoor variables it is almost impossible to correlate with the controlled indoor training.
Indoors I have an H3 and ERG, music and 3 fans, outdoors in Fla. I have 20 gears, headwind/side wind/tailwind, bridges, corners , other riders, cars etc.
I guess if you had a PM and a place outdoors you could set an outdoor FTP like some do ,. depends on level of involvement and how far down the rabbit hole you want to go.

My average power inside often is much higher than outside especially with the long continuous threshold (eg looking glass +1) and sweetspot (eg Roan High +2) workouts …

But according to Bicycle Speed (Velocity) And Power Calculator if you were in the drops you could hold 18 mph with around 124W

Kickrs are generally known to be pretty accurate. So unless you have a complete dud, I doubt you’ve got a 250W FTP and the kickr is say it’s 136.

At 131 lbs I imagine you’re also pretty short and narrow so you’d be pretty good aerodynamically. 136 sounds low to me to be cruising at 18mph but I’m also 6ft 180 with broader shoulders so not a great reference point.

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If you can control for motivation, there shouldn’t be much difference other than cooling.
I use the same PM inside and outside, and I see very similar numbers. Power, RPE, HR, they are all the same, at least as far as I can tell.

I mean, to some extent it doesn’t matter what the indoor trainer is saying provided it is consistent - as it’s just a number to guide your training.

But it does sound fairly low. 136 also sounds low to me for cruising at 18mph and I’m 5’8", 155 lbs (ish). Just looking at my ride from the weekend, which was a bit wind affected (and we have heavy roads in the UK), I did around that for a long period for around 165-170 watts.