For cars which have shift lights in cockpit, for example, shift alarm will go on with ingame shift light at the same time.
For cars which have no shift lights in cockpit, shift alarm will go on with shift alarm in gear/RPM HUD at the same time, this also applies to 2000cc Subaru which has shift lights in cockpit, because the shift alarm in the HUD goes on earlier.

NOTE: I have no programing/coding skills, all these results came from my manual testings of stepping on the throttle and observing if ingame shift light and dashpanel shift light go up at the same time, so these results are not 100% accurate, and if you feel the shift light goes up a little earlier or later, just change the %, 0.1 or 0.2 should be enough.
My english isn't good enough, leave a comment if there's something confusing you.