5% would be 73.5 mph, you are at ~10.3% off.

You might check and see if the tires and rims are stock or if they have been changed in the last 28 years?

Speedos in that era are typically problematic. My 85 wobbles so much I usually couldnt tell you exactly how fast I am going.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)