The speedo get substancially off the further past 75-80mph you go.
close estimates3-4% at 50mph (48mph)
5% at 75mph (72mph)
7% at 100mph (93mph)
9% at 125mph (115mph)
11% at 150mph (134mph)
Tire diameter can drastically effect indicated as well.
I have hit as high around 150mph indicated and the OBD-II showed 136mph max speed. I have stock tire diameter (minus a tenth or so for wear)
It was still pulling well at that point too...
Closed road, professional driver, contents may settle, disclaimer of course...
