I have always blamed the veglia speedo on my Le Mans and T3 Cali for being erratic, jumping from 20 to 40 and general vagueness (speed cameras have always been a challenge) Once in 5th gear it’s not too far off the mark.
Just re built a G5 which had no clocks so bought a Daytona Valona speedo which comes with an electronic sender that screws into the speedo drive in the gearbox. All set up and it’s just like the Le Mans, maybe even worse.
Have now bought a sensor for the front wheel.
So for years I have maligned my Vaguelia, sorry Veglia clocks and after all it wasn’t their fault!

PS: unless your speedo drive works properly (do they?) and you buy a Daytona speedo don’t pay extra for the sender, get a pick up.
PPS: if you want a sender I have one!

The Vaguelia clocks on my Spada and Cali 3 are actually quite steady and reasonably accurate when compared to a sat nav, maybe reading 5% high, but most vehicle speedos are similar. Keeping the cable well lubed helps to get a decent reading from them, but use grease rather than oil and don’t apply it to the top 6 inches or it will get into the speedo head and that will cause eratic readings.
In contrast the Veglia instruments on my T140 Bonneville are all over the place and flicker around like mad!

There is a requirement that speedos have to read high and not low, so that you do not break the speed limit when you think you are within it! I think it is in The Road Vehicles (Construction and Use) Regulations that the speedo should not read over 110% of the actual speed and also not below it.

I have just ordered a cheapy Chinese GPS speedo for my Scrambler project. It will be interesting to see how well that works.