I already queried Allan about this when the shaft speed appeared, and to be honest still unsure of the reply. This was over a year ago I askedRob Stevens wrote: The ecu could work the latter out if it had both driven wheel speed sensors fitted, perhaps Allan will add this some time for S80 users?
He replied that "shift cut is normally used on a sequential box, not an H pattern. If you have a sequential box you will use a gear pot and therefore will not need to measure gear this way. There is method in the madness"
To me, the sensible option would be via wheel speed, although yes there is a possible chance of inaccuracy during wheelspin of one wheel.
But if you measured a single driveshaft on say a fwd car, that same problem would happen anyway.
the only way to get a truly accurate relationship between shaft speed and gear/engine rpm is to monitor propshaft speed, or in case of fwd, actual diff input shaft speed ( ie mainshaft )