Seven Gravel - IF of .89 for 6hrs - big diesel or wrong FTP or other?

[toyman]

I think maybe a combination of both?
Rather than the 20 minute test, I would suggest doing one the progressions from Emperical cycling:

Kolie Moore's FTP test protocol

I’ve seen some mention of this in some other threads, but I’m wondering who here as done one of Kolie Moore’s FTP test protocols, and how it worked for them. I’m thinking of giving one a try and wanted to gather other’s experiences and how they constructed a workout based on his descriptions. In particular, what would be a ‘gradual’ increase in power for the baseline and progression 1, which is what I would be experimenting with.

Since your FTP number is likely low (and almost certainly not high), trying one of the longer progressions with your current FTP will tell you if it is low. Note that in this case the tests won’t actually give you a valid FTP because you started at a very low number, but at least you will get a good idea that it is low.