The opposite IMO mate.
Speedos are designed to overead!
So you're doing indicated 38 for example, taking the piss a little.
Old cable driven speedo typically 10% reading over, takes it down to about 34, so with 10% +2 you're safe, yet thinking you're doing 38.

Hope I explained that well?!
My diff driven is 2/3 mph over at all speeds, old cable driven could sit at 80 on Mway happily, doing 72/73, so be very unlucky to cop it for that.
Indicated 84/85 before getting done if 10% +2 is in operation.
In hgv's you have zero overead, which is a fcuker, as overead has probably helped you once or twice over the years.
Was in one I was caught doing 38, in old car may have been alright.