> Anyhow, my TR4 is a 1964, standard transmission. The numbers on
> the facia of
> the speed appears: SN 6325 OOA 208182 and 1184.
The important number as far as 'correct' operation is the last one. It
indicates how many times the speedo cable turns for each mile traveled. Of
course it may not be 'right', unless your tires are the same rolling
diameter as the factory ones.
> Out of curiosity, to enrichen my life why, as some
> folks informed me privately, that there are two types of speedos, one for
> standard and the other with an overdrive. Why the difference?
That seems to be a common myth, it is simply not true. Since the speedo
drive is taken after the overdrive, it depends only on the final drive ratio
and the tire diameter, it doesn't 'care' what gear the transmission is in,
including overdrive.
There is a tiny kernel of truth to the myth, in that the documentation
indicates that the optional 4.11 final drive ratio was only available in
combination with overdrive. But the vast majority of cars with factory
overdrive still had the standard 3.7 ratio final drive and hence the same
speedo calibration as non-OD cars.
There is one other 'gotcha', the later TR6 transmission will interchange
with the TR4 transmission. However, it has a different speedo drive ratio
and so if you install one, your speedo winds up reading some 20% low. Other
Triumphs also used the same gearbox, and likewise had different speedo drive
ratios, so buying a used transmission is something of a crap shoot unless
you know exactly what it came from.
Randall
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