Construction and use regulations allow a speedo tolerance of -0% to +10% so all speedos should overread. To accommodate manufacturing tolerances most are towards the 10% error but 39mph at 30mph is outside the limit.
True but since its never checked by officialdom, that doesnt matter.
I would be a bit cautious comparing with a satnav because they arent htat accurate on speeds paricularly if unudlations and hills are involved. Their speed calculation is the distance between two calculated points on the geoid and the time difference. That said the satnav wont be 30% out or anything like it. Check wheel and tyre sizes. Check if the speedo has been replaced - does wonders for resale value sometimes!
