My speedometer was optimistic by about 15 percent. So if it indicated 90, it was actually going about 76. Stock size tires. As someone else observed on theirs, you could take the km/h reading and divide by 2 to come pretty close to the actual mph.
I hated the speedometer because I could hardy read the MPH red markings anyway. So I used a Sharpie and Dymo labels to mark 20, 40, 60, and 80 around the rim of the speedometer at GPS-verified speeds.
I never checked the odometer for accuracy.