From: jgd (John De Armond)
X-Source: The Hotrod Mailing list
Date: Apr 1992
Subject: Speedometers

>i just got through correcting the speedo in my LeSabre.  it has a
>TH400, and from what i was told, the same info applies to the TH350
>
>The speedo gear is on the side of the tranny behind where the speedo
>cable gets screwed in.  it is a plastic gear behind this housing that
>reads the output shaft speed.


Just a small note about an alternative method of dealing with speedo
error.  A speedometer repair shop will recalibrate and/or compensate
your speedometer for prices that seem to average about $50.  If the
speedo simply needs calibrating, they'll remove it and do it.
If you've done something that requirs compensation, they will install
a small gearbox in the cable and install appropriate gears to
make the speedo read right.  Unless one just likes to rummage around
for the proper pinion gear and/or one does not assign a value to his
time, getting the speedo shop to do the deed is about the cheapest
and surely the easiest way out.

John

From: emory!merlin.gatech.edu!ucsd!btree!hale
X-Source: The Hotrod Mailing list
Date: May 1993
Subject: Re: Comments.....
X-Sequence: 5345

> Anyway,
> the car seems pretty tight and is a lot of fun to drive.  So, I found an old
> magazine that listed the 0-60 time for this year vette at 9.5 seconds and
> decided to see how mine compared.  Lacking any sophisticated equipment, I
> tried just using my stop watch and watch the speedo.  Well, I tried this 3
> times in a row, from a standing start (on wet road no less and my tires are not
> that hot) and I got 7.2 seconds.. granted, my speedo can be off, but 2 seconds
> off the factory time seems a bit much for minor changes that I've made.

I have wondered about the accuracy of a speedometer under acceleration
conditions.  Mine seems to read about 5 MPH high at 60 under full
throttle conditions.

I built a little gadget to measure 0-N MPH times.  It consists of
a DC generator mechanically coupled to the speedometer cable, a box
of electronics which contains two comparators, a time base, and
a counter.  One comparator has its threshold set to about a millivolt
above zero and the other comparator is connected to a variable
reference voltage.  The first one starts the counter and the second
one stops it.

To use the box I cruise at the desired set speed (60 MPH, for example)
and adjust the reference voltage until the comparator output is just
toggling (it sees the ripple on the generator's output).  Then I get
set up to make the acceleration run, press the reset button, and go
for it.  I find that the comparator output goes true well after the
speedo reads the set speed.  For a 0-60 test the speedo usually reads
about 65.

Has anyone else experienced this acceleration-dependent behavior
in a speedo?

[Absolutely.  My speedo expert tells me most speedos are under-damped
intentionally so that people will err or the side of NOT breaking the
speed limit.  JGD]

Bob Hale                                      ...!ucsd!btree!hale
...!hale@brooktree.com (preferred)

From: Bob Hale <emory!brooktree.com!hale>
X-Source: The Hotrod Mailing list
Date: May 1993
Subject: Speedos and sensors
X-Sequence: 5349

I had some e-mail asking about my 0-60 meter and thought that I would
pass on the question and answer here.

The question was how I manage to connect both the speedo and the
sensor at the same time.

Quite a few years ago I happened across a closeout sale of MPG meters.
These meters had a clever arrangement - the vehicle speed was sensed
by a small DC generator which had a male speedo fitting on one end
and had a female fitting on the other.  I just used this generator
in series with the speedo cable.

I wish I knew where to get more of these; it's a really handy thing.
The company that made it had a mold built which has the appropriate
threads for a speedo cable on the one end, and the other end takes
a plastic collar which is internally threaded to match the tranny.
That mold cost them a lot of money; too bad they don't seem to be
in business any more.  I would like to buy some empty housings.

If you have access to a lathe then you could make such a housing
in about 1/2 of a day.  The bearings and square shaft would take
some extra time.


Perhaps someone could comment about the availability of modern parts
which would do the same thing.  Perhaps a pulse counting sensor
could be modified.  The hard part is getting the housing; changing
the guts should be easy.

Bob Hale                                      ...!ucsd!btree!hale
...!hale@brooktree.com (preferred)


[I'm not sure about analog generators but digital interruptors are commonly
available because they are used on many cars.  A good source for new ones
is a speedometer repair shop.  This would be only slightly more complicated
to use.  A single tach-to-analog chip would do, as would a simple
microprocessor.  JGD]