John Bartholomew wrote:
>
> I have just taken my 74B out on it's first test drive since rebuilding the
> engine and tranny. I am puzzled now, why my speedo reads low. Assuming the
> tach reads accurately, in O/D top gear at 3K I have an indicated road speed
> of 50MPH, not a tad over 60MPH as I had expected.
> I do not recall any problem with the speedo before the rebuild, but then I
> have only driven this car a total of about 2 miles, ever.
> It is possible I missed the fact that the indicated speed was off when I
> drove it, as it was just around a residential block to check that the O/D
> and tranny worked OK. The O/D did not engage on my test drive so I checked
> out the electrical side and all was in order, so the assumption was that the
> mechanical side had the problem. This car has sat for 16 years under cover
> but is in great shape otherwise.
> My fix for the O/D was to swap it with a rebuilt unit from a 79. Both units
> were identical from the exterior, however, now I am thinking that the drive
> ratio to the speedo pinion must be different. Could this be so? Are there
> any people out there with the necessary knowledge to comment. I would
> appreciate all offers of help with suggestions. Mr Twist are you listening?
> ----------------------------------------------------------------------
> E-mail jdb70@juts.ccc.amdahl.com John Bartholomew Amdahl San Jose CA.
> 408-746-6361 07:00-16:00 PST. ** USUAL DISCAILMERS APPLY **
> 1969 MGBGT;1974 MGB;1976 MGB-RESTORED;1979 MGB;1972 MGB daily driver.
> ----------------------------------------------------------------------
Just installed an OD in my 74B so I had to do a little research on this
topic. The speedo gear drives are different. There are 2 types of "LH"
OD's. One has a black nameplate on it requiring speedo #5230-08s. The
other is a red plate(76 or 77? to 80 models) which requird no change in
the factory equipped speedo(don't know the # off hand). I'd say you have
a choice of having your old one properly calibrated or finding a speedo
out of a later model rubber-bumper B. There was a great speedo
cross-reference chart published in the MG Driver (NAMGBR publication)
last year. Hope this is helpful.
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