Speedo Error

Took the 77' KZ1000A1 out on the open interstate today following my son in his 2014 sedan... his, all digital dash and gps... etc etc...
The plan was to see what my speedo reads compared to his super whiz bang, woof, woof 2014 Ford.
At his indicated 55mph, I'm reading about 58mph
65mph I'm reading almost 69-70mph
75mph I'm reading almost 81-82mph
85mph I'm reading about 93mph
95mph I''m reading about +103mph-104mph
105mph I'm reading almost 115mph.
He would not go any faster then 105mph but I could still zoom right past him at 105mph in 5th gear and get the KZ1000's speedo up to about +135mph before backing down.... and it felt like it was still accelerating too.
Does nearly an 8% to 10% error at a little over 100mph seem reasonable or are these old instruments just worn out at only +35,000 miles?
Front spoke wheel is stock and the tire is a mid worn 100/90-19....

Typical error.

I tried to get mine to stop jumping, now it's slow. 55 means 65 and so on.
Steve

Supposedly, the magnets inside the speedometer become old over time, yielding an incorrect indicated speed. Others have reported speedometer error on newer models --- I guess the magnets could be the culprit on those as well. ???

Z1-Resto