In top I know each 1000 RPM = 20MPH (checked with GPS) but speedo reads:

 

Real 30 = Speedo 32

Real 40 = Speedo 43.5

Real 50 = Speedo 54

Real 60 = Speedo 65

Real 70 = Speedo 76

 

I just add a bit more to shown figure but can this be adjusted from inside the speedo head or is this error 'baked in' by the spring strength used? Anyone ever tried to make them read true(ish)?

 

Cheers.

@Mr105LUX - Your figures are sort of typical for a car of that age. Although we used to calibrate the other way, by doing indicated $speed over a known distance, and timing it.

Cheers Ken. I never really used to care about the speedos being off a bit, but especially for my mum - who uses her Estelle on her commute along fast but narrow A roads, other drivers are now faster than ever and are doing at least the limit, so a slower old car like the Estelle 'has to be got round'. She knows about adding speed to that displayed, but honestly I don't think that this is something you should be thinking about at higher speeds. So I want to get this set up as good as possible for her so she can tell at a glance what speed she is travelling. I may experiment on an old speedo head. I'll report my findings.

 

My 105 has 16.6MPH / 1000RPM in top so 3000RPM = 50MPH. Other speeds are harder to work out.