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.
The law allows more than that Chris, it's up to 110% + 6.25 mph over read in the range 25 to 70 mph. A cars speedo could legallly be indicating 50.25 mph when actually travelling at 40 mph
