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
|