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Re: Does the OD [now speedo calibration]

To: "David Councill" <dcouncil@imt.net>,
Subject: Re: Does the OD [now speedo calibration]
From: Max Heim <mvheim@studiolimage.com>
Date: Tue, 21 Dec 1999 12:41:08 -0800
This talk of recalibrating the speedometer by changing the face puzzles 
me. Since the speedos are designed with a certain speed mark at the 12 
o'clock position (usually 60mph), and the total sweep wouldn't change 
(stops at either end, presumably, but see below), where is the latitude 
for recalibration?

Looking at the fascias in Clausager, all the Bs shown have 60mph in the 
12 o'clock position, except for the 1980 with the lame Fed-spec 85mph 
speedo (before anyone jumps on me, the Cs and V8s have 70mph straight up, 
since their speedos read to 140mph). They all seem to have a 270 sweep, 
also, though that is difficult to tell precisely. It is difficult to 
determine which, if any, of the cars shown have OD, but I would have 
expected that any differences in final drive ratio would have to be 
compensated for by either the speedo drive pinion or the internals of the 
speedo gauge itself (since the gauge markings are all apparently 
identical). 

The only explanation I can think of is that the new (or alternate) face 
has 0mph in the same position relative to the internal works (obviously), 
and then has a wider (or narrower) sweep, with the type markings aligned 
so that they are still horizontal when 60 is in the 12 o'clock position; 
and that you rotate the entire unit when you put it back into the car, to 
keep 60 straight up. I guess this seems to work, except that the sweep 
would no longer be exactly 270. Perhaps the differences are so subtle 
that they aren't visually apparent.

Anyway, this is interesting news, in that buying expensive speedometers 
merely for this reason becomes a non-issue. Why didn't any of the big 
vendors tell us this? <g>

David Councill had this to say:

>This would seem logical. But I thought the internal gearing was different 
>on the speedos originally. Even if so, a recalibrated face would be an 
>easier fix. When I first changed my o/d, I just put a couple of small 
>pieces of scotch tape on the side and then made marks for 60 and 70 mph 
>which I determined by a long highway drive, timing the intervals between 
>mileage markers.
>
>David
>67 BGT
>71 BGT
>
>At 12:59 PM 12/21/99 -0500, Kai M. Radicke wrote:
>>The speedometer does not need to be changed, I repeat... the speedometer
>>does not need to be changed.  The only difference is the face of the two
>>speedometer, the internals are all the same.  It is a simple swap... and
>>much less costly than purchasing a new or used unit.  Not to mention that if
>>you have a sad looking speedometer, a new face can really bring the life
>>back to it!
>>
>>Call Nisonger / http://www.nisonger.com/
>>
>>They'll have the face in stock.
>>
>>Again this is another "ask me how I know" learning experiences ;-)
>>
>>Cheers!
>>
>>--
>>Kai M. Radicke -- kradick1@ic3.ithaca.edu
>>'74 Triumph TR6 -- http://www.pil.net/~felix/
>>


--

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.


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