Hondata

Looking at a transmission rebuild for FK8, and am considering swapping out the FK8's stock 4.111 final drive with the 2.0T Accord 6MT's 3.842 final drive.

Since the ECU calculates VSS from a sensor that reads off the 3rd gear countershaft teeth (per Flashpro Manager helpfile), going to a taller final drive ratio will slow the countershaft speed by about 6.5%, so this in turn would seem to throw off the VSA and ABS systems. My question is, does the speed adjustment factor on Flashpro Manager for the FK8 handle this difference internally with the ECU if the correct value is entered, or will this require some sort of external module to correct the VSS signal for the ECU?

No, speed correction is more for a difference in tire sizes as there is no way to alter the gear ratios for calculations.

UPDATE:
The final drive ratio swap (using Accord 6MT final drive in FK8 gearbox) has been completed and everything seems to be working fine. It turns out that all that's needed is the speedometer correction factor. In this particular case, a total correction factor of 8.5% was used to account for the taller final drive gearing and ~2% larger circumference tires that are installed on this car. So far, rev match and everything seems to be working normally but I will report back here if anything turns up down the road.