Not sure re different Speedo's for O/D etc but was having a problem with
mine in that it was over reading i.e. saying that I was going faster and
travelling further than I was.
I have since sent it off for recalibration to a firm in SE England (UK).
What I had to do was measure the tyre circumference, remove the speed, jack
up the rear wheel, turn it twelve times and count how many times the speedo
drive turns itself.
If you're interested I can supply the address/phone no. of where I sent it
but can't comment on the quality of the work as I'm still waiting for it's
I have a 1972 MGB with a 5 speed V8 conversion so it may be different for an
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