1963 Fourgonnette. I have done an engine / gearbox swap out from the 425 to 605 with a 'new' drummer gearbox.
All gone Ok including 6 to 12 volt conversion. The Original speedo now reads about 17kph slow (about 10mph?).
Any ideas ?

go 10mph faster?

Similar here - my 350 now has a disc brake gearbox and gives the following readings:

40kph (which should be 25mph) is actually just under 30mph (in fact, I use the needle's position at 'just past the 40' as 30mph)
100kph (supposedly 62mph) is actually around 68mph on the satnav

It's about 10% out - you could mark the speedo to record the new readings or just try to remember it (I've opted for the latter) although I did read somewhere that a correctly working speedo may be part of a future MOT, so we may have to actually do something to correct it at some point...

I doubt it would affect a vehicle of that age if they did introduce such a thing.

Gaffa over it. Problems go away!

I adjusted mine by taking the face (glass) off, driving at 30mph (sat nav) and then stuck the needle back on at 30mph. Worked a treat

Great,but sat navs read slower don't they?

Yes but by the time you dont quite get it on perfectly cos yoir driving its ok.
Also sat navs read fine its cars read faster

oh

No, sat nav systems tend to be bang on with speed.

Speedos on modern cars under-read, usually due to the manufacturer playing it safe, but with the 30-odd years of wear we'll all have in the mechanisms, along with plenty of engine and gearbox changes thrown into the mix its anyone's guess as to just how accurate our 2cv speedos are...