40/27 |3 -3 1>
680.4487 cents
https://en.xen.wiki/w/File:Jid_40_27_pluck_adu_dr220.mp3 sound sample
The grave fifth of 40/27 is the interval between 9/8 and 5/3. It is 81/80, a syntonic comma, flat of 3/2. In some contexts it may be considered a wolf fifth. In meantone, of course, it is equated with a perfect fifth.