Speedo recalibration - TR4

[asj@dsbc.icl.co.uk (Adrian Jefferies)]

Hi,

My speedo reads incorrectly since the overdrive was fitted. I understand
it is possible to recalibrate the things but I cant figure out how.
I've looked at one that was in bits & all there seems to be is a spinning
metal bar (driven by the cable) within a metal cap (the needle).
I've got the TR4 early domed instruments - cant remember if they are
Smiths or Jaegar though.

Could some kind soul enlighten me?

Regards, Adrian
        asj@dsbc.icl.co.uk