$42 VSS Filtering in REAL TIME SERVICE ROUTINE B

I have noticed that the mph in my data logs doesn't match my speedo. I used my GPS to determine that the speedo is accurate to within about 1 mph. The TP logs actually have the MPH about 4 mph slower than what my speedo registers. I decided to look at it a little closer tonight. The MPH ( I think) is calculated in the Real Time Service Routine B. This is part of the prom, not the calibration, but I am curious about how it calculates MPH from the VSS signal without knowing tire size or rear diff ratios. Has anyone spent any effort looking at this piece of code? The hac I have is not commented very much at all, so no easy hints there.