I think you meant most speedos read OVER what you're actually doing (ie optimistic), which is usually true, yes....unless someone has fitted different size driven wheels or tyre profile.
I use my satnav when driving through areas with cameras, as it's very accurate.
Naturally, being an old bloke I only speed up when pulling out to overtake 10 or more slower cars....mainly driven by women of all ages, but normal female timid driving habits.
Been driving average 20,000 miles + a year for 42 years, only ever had 3 points, which came off in 1973.
In other words, yes I have been very LUCKY.