I'm not sure about your particular year, but for many years, an OD speedo
has a different gear ratio than a non-OD speedo. The easiest way to
correct it would be to get the right speedo for the gearbox you have. The
more complex (but also more accurate way) would be to take out your
speedo, count the number of turns the speedo cable makes in 52' 9.5", send
the information off to a speedo shop like nosinger - and have them gear it
properly.
I'm not sure about the speedo pinion on the gearbox - mesching gears have
to have the same dimetral pitch, and the spacing has to be right, so
there's a pretty good chance that you can't just change speedo drive
pinions. As far as cable lubricant, I don't think it's just graphite,
because its pretty sticky and nasty stuff on that cabel. People use all
kinds of things - I've even heard WD-40.
For the jerky/wobble problems, it is probably the cable. Chances are good
that there is a bend in the cable that isn't real smooth, and that is
causing a problem, or that you do (or don't) have an angle drive and you
shouldn't (or shoul) to take out one kink right by the gearbox.
Phil Bates
67 MGB with overdrive
> So my mechanic, about a year ago, converted my 78 B to overdrive.
> Hooray! I now have two problems with it: the speedo is now very jerky,
> tending to settle down a bit more at speed (around 20 mph, it jerks
> between 0 and 40, but only oscillates by 10 mph or so at 70 mph).
> Second, it consistently reads about 10 mph (maybe 10%, actually) lower
> than my actual speed (as measured with a GPS).
>
> For the first problem, do I correctly remember that powdered graphite
> is a good lubricant for the cable? Funnily enough, Dad still had the
> tube left over from a Pinewood Derby, so I'm set there.
>
> For the second problem, my best guess is that the OD has the wrong
> speedo pinion on it (Moss parts 462-610 or 462-615 for OD, but I
> suspect he may have just left the original non-OD pinion on, if that's
> possible). Would that be at all consistent with reading low all the
> time? And am I correct in thinking that which pinion you use depends
> on the marking of the OD unit itself?
>
> (or should I just give up and go back to my mechanic and tell him it
> just don't work and he should fix it? it's been a bit of a saga with
> this speedo...)
>
> Thanks,
> Derek (78 B in Amsterdam, and now a 2002 Saab 93 diesel for the commute)
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