Speedo 5% error.

[RatherBNtheWoods]

Since the last ECM software update(flash) my speedo has been 5% high. I fought and argued with the dealership, and lost. Figured I'd burn that bridge later. Then the rear ABS sensor(speed sensor) failed. ABS light came on and the speedo started reading dead-on. Replaced the sensor yesterday, light went out, ABS now works correctly, and the speedo went back to the 5% error.
WTH?