Tony :
I can verify that the speedo head was, in fact, different for the 4.10
rear end, because I have an apparently original speedo with a 1325
calibration in my car (unfortunately, I don't seem to have the 4.10 rear
end to go with it).
And, having kept an eye out for both for many years, I believe I can say
with some authority that 4.10 rear ends (especially to fit TR3s) are
much rarer than TR3s with overdrive, at least in the US. Careful
reading of the factory literature shows that the 4.10 was available ONLY
if OD was also fitted, but OD was available without the 4.10. In the
US, with our relatively high proportion of 'high speed' roads, the OD
made a lot more sense than the 4.10, even in the late '50s.
In fact, most of the 4.10 rear ends I've seen appear to have been
converted using gears from another model car. I think this explains why
most cars with 4.10 gears seem to have 3.70 speedos.
Randall
59 TR3A TS39781LO daily driver
Lakewood, CA
Tony Rhodes wrote:
>
> Message text written by "Michael D. Porter"
> >The problem is likely with the speedometer installed, or with the rear
> >end ratio. Twenty percent is the magic number here--that's approximately
> >the difference in engine speed between overdrive and non-overdrive cars
> >at a given road speed. Since Triumph put a numerically higher ratio
> >differential in all US cars with overdrive, using the overdrive to bring
> >>engine speed down to that of non-overdrive cars, the speedometer would
> >read 20% high if the speedo were non-o/d, and the o/d diff. were
> >installed.<
>
> You bring up a number of interesting point and one (possibly 2) fallacy ;)
>
> Fallacy: The OD makes a difference in what the speedo sees vs. road speed.
> TRUTH: Since the speedo drive gear is AFTER the OD, it sees the drive shaft
> RPM which is independent on geear & OD selection, and only dependent on
> road speed and differential ratio.
>
> Possible Fallacy: ALL OD-equipped cars had the 4.1 diff. Not so, as far as
>I can see.
>
> Consider: I have NEVER found a speedo calibrated for the 4.1 diff! I am in
>the process of
> being mystified about this one. MAYBE the speedo drive gear is changed when
>the 4.1
> was fitted. This would fit some of my friend's situation....
>
> Your point of the 20% error being EXACTLY (almost) the difference in 4.1 vs
>3.7 is interesting.
> I would love to pull out the speedo drive gear. I would LOVE to trade him
>mine since I
> DO have the 4.1.
>
> One possible consideration: NOTHING at all was changed when he got the
>tranny rebuilt
> at TRF, except the tranny. Previously, the calibration seemed OK, then it
>was in error. Seems
> like the tranny is the only variable. Of course we know there are actually
>infinitely many variables
> in reality. I am trying to limit the search at first.
>
> -Tony
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