VP inside and PM outside: calculating ftp


I’m getting a power meter for the first time (yay), but am planning on using it outside only (I have a dedicated bike for my training)

Since I use a dumb trainer, I’m used to virtual power

I don’t feel like testing my FTP again with my PM (I did a ramp test recently), and was wondering if it made any sense to use what I know the gear ratio and RPM are to reach my FTP and look what the reading is on the PM ?

That way, I can use one FTP given by VP inside, and another outside.

I figure that power is power, but maybe I’m missing something…

Unfortunately, the same gear ratio and cadence inside and outside will yield super different results in power output, due to a multitude of factors, such as resistance on your indoor trainer versus the lack thereof outside.

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What kind of PM is it? If its something that you can put on your inside bike (like pedals or a left crank arm if the cranksets are the same) then you can install it on your inside bike and dual record the power. If the two numbers are the same (or atleast within 3-5%) then you’re probably fine not testing again. But then if the readings vary by too much then you are still left with the issue of all of your data being all over the place.

But yeah…as @IvyAudrain said…you can’t compare the same gear ratio and RPM on your trainer to outside. Trainers have artificial power curves and they aren’t always designed to match outside. And even if it was designed that way it can’t take into account the different rolling resistance, wind, body position, etc.

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I meant on my indoor trainer ?

Sorry, it really wasn’t clear !

I’d put my PM on my bike inside just once to see what kind of reading it’d give with the usual conditions

Thanks for the answer ! Yes, they are pedals

I’m aware it’s always better to have one coherent set of data, but i’m really just a beginner and want to have fun with it and learn how to pace my efforts or push myself. I won’t race or anything, i just want to be a better cyclist

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Then that should be easy to pop them on the inside bike and just do a quick ride that goes up and down the power curve and try to see how well the two powers align. Maybe record one on a Garmin and one on a computer then look at them afterward.


I can definitely do this, thanks for the tip !

When I was at your stage, I was in a similar boat. I had an indoor FTP on virtual FTP of 485, and outdoor FTP on powermeter of 286…

I’d get a new outdoor PM. That one is obviously wrong.