Tony -
I made a transmission swap sort of similar to yours (3-main, non OD trans
to 5-main OD trans on a 3-main engine) on my '63 B many years ago and found
the original speedo was then about 20% off. I looked for a used speedo
that belonged with the new trans, but found those had a smaller diameter
and would not fill the hole in the dash. I therefore had a local shop
recalibrate my original unit; it was was very successful. However, since
you have apparently stayed with the same vintage trans and just added one
having OD, you probably could find a used speedo designed for the new
trans.
If you prefere to recalibrate your present unit, look for the small, four
digit number just below the center of the speedo face. It gives the number
of revolutions of the cable per mile that the speedo is set up for. As I
recall, the numbers for the various MGB units I looked at were in the range
of about 1000 to 1400. If you can find the value your new trans is set up
for (it's in the workshop manuals - I could probably find it for you if you
can't), tell the shop to convert your speedo to that value, as opposed to
letting them measure the adjustment needed by putting the car on a
"treadmill". When the shop tried to measure the correction I needed, it
was off by about 5%. I took it back and told them to set it up for exactly
so many revs per mile and it was perfect.
Russ Wilson
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