1982 speedometer gauge

Is there a calibration adjustment on the electric speedo? I mounted a GPS on my bars, and discovered my speed above 65mph doesn't read correctly. I'm actually cruising at 73 when my gauge shows 65, and it gets worse the faster I go.

Did you change fron tire size(bigger/smaller)?

Size as specified, with minimal tread wear.
Thanks...

hmmm, interesting. Usually they are off in the other direction, that is, speedo usually reads 5-10% faster than actual speed, I'll bet a speedo shop could recalibrate or repair it, but may be $$$$$.