All Gen 3 SHOs get their info from the ABS module (below from Helms)
The speedometer operates in the following manner:
The speedometer (17255) receives a speed/distance signal from the
transaxle-mounted vehicle speed sensor (VSS) (9E731) (except 3.4L SHO).
1st 6.94 mph / 1k RPM
2nd 12.57 mph / 1k RPM
3rd 19.23 mph / 1k RPM
OD 27.72 mph / 1k RPM
I remember 7, 12.5, 20, 28 for my mental calc when I drive down the road, that is close enough.
My speedo is 130/127 ~ 2.36% high. using Auto Tap to get direct OBDII readings.
For most SHO owners if their speedo is like mine it will read 2.36% high.
My summer tires are only 25.2" tall, which makes them 1.62% shy of OE.
.9838 x 1.0236 ~ 1.007 so the two errors off set to a large degree and my speedo is more accurate with 235/45-17's than the factory tire. It is with in less than 1%.
My 144 may have been 142.9934 MPH. since I don't know how to do decimals in Roman numerals I'll keep CXLIV ;-)