There are two issues here, the accuracy of the speedo, and the accuracy of
the odo, separate devices. The small 4-digit number above MPH on the speedo
dial is the number of turns at the wheels per miles on the odometer. The
speedometer, an analog device can be calibrated by moving the pointer to
indicate true speed. The odometer, however, is fixed by the internal gears,
and can only be "calibrated" by changing the gearing. Lister Anthony Rhodes
has produced a great document describing the workings of the speedo -- go to
http://ourworld.compuserve.com/homepages/arhodes
follow the link to "Smiths/Jaeger Speedometer Repair Manual".
The key thing is not the OD itself, but the fact that the rear end was
(sometimes / usually) different ratio which directly affects the turns per
mile, and the accuracy of the odo. You didn't give the year of your car; the
first few years of the TR6 (I suppose the years with the "A" type ODs) were
the same as the 4/4A, ie, 32 tooth pawl wheel.
My TR4A has the 4.1 optional rear end w OD and my speedo is a "1312", where
the 'standard' 3.7 speedo/odo was a "1184"
It's possible that there were "1000" and "1120" variations of later 20-tooth
pawl TR6 speedos for the "J" type ODs with different rear ends --
HTH
January
66 TR4A CTC74217LO
------------------------------
Date: Thu, 03 Oct 2002 00:26:33 -0400
From: Patrick Bitton <pbitton@axess.com>
Subject: TR-6 Speedometer for OD Tranny
Someone on Ebay is selling a speedometer that claims it's for OD
transmissions. I asked him the difference and he told me that "Revolutions
are different for the OD compared to the non OD. One is 1000 and the other
is 1120." Is this true? Do I need to replace my speedometer to get a right
reading? It seems that when I feel that I am going 50km/h, the reading is
about 30km/h. Has anyone experienced this while doing this conversion?
Patrick
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