When my l322 td6 was killing an abs sensor i could get upto about 10mph then upto 70 before it would register a change in speed it was delayed and laggy more so when wet, they send speed from rear left sensor through the abs pump to the cluster,
I dont know how the p38 prosses wheel speed to the speedometer though im afraid, if you could get diagnostics on all 4 corners and see they read the same,
Possible a wheelbearings got play and upsetting the relucter ring reading,
iirc the ABS module counts how many teeth on each reluctor ring pass per second and average it across the 4 wheels. I cannot remember how many pulses there are but Wammers put it up a few times. That rate goes to the BECM and a signal is sent to the speedo to display the current speed. On the back of the speedo are a couple of pots that calibrate that signal so it lines up with the marks on the speedo.