I was one of the folks to talk about '6 speedo units. I believe the
discussion is the same for the '4/4A. There is a fixed tooth counter
gear on the tailshaft of your tranny. The tooth count is the same for
the non-OD and OD transmissions. The speedo unit 'counts' the number of
revolutions the tailshaft turns and converts that to a speed indicator
and odometer counter. It is the responsibility of the speedo unit
calibration to get the conversion right. Unfortunately, changes in diff
ratios and/or tire size will affect the accuracy of the conversion,
causing an under/over speed depending on the final drive rations
achieved.
Scott.
> I've even heard rumors that the factory installed the incorrect units in
the
> later '6s so the U.S. folks would think they were zipping around (can
> you conform John Mac?).
Nice little rumour, Scott - but I don't think they went that far to get a
sale! Oh my, the inventiveness of car salesmen - bet they were from MG!
I've been watching these speedo threads recently and wish there was
something I could add to help people. In fact, I've already had a delve in
the archives at HMC but can't come up with anything of any real practical
help. All I can say (and this muddies an already frustrating problem) is
that a different speedo was installed when TR's (4 and 4A) had line fitted
Michelin X tyres. In my days, they were production fitted optional extras
and I well remember the 'production tally card' would read
MICHELIN X TYRES
MICHELIN X SPEEDO
This was because the Michelin tyre had a sufficiently different rolling
radius to exceed the margin of error +/- of the conventional tyre, whatever
that was. I think the MoE was about +/- 5%. I'll try and see if I can dig
something out of dusty paperwork tomorrow at HMC - but no promises.
John Mac
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