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How can I figure out my diff ratio without using fancy tools?

To: "Charlie" <c2h60_@email.msn.com>, Triumph List <triumphs@autox.team.net>
Subject: How can I figure out my diff ratio without using fancy tools?
From: Tony Rhodes <ARhodes@compuserve.com>
Date: Tue, 9 Mar 1999 20:21:47 -0500
Message text written by "Charlie"
>ended up with one tire revolution = three and a half engine revolutions.
Does this mean I have 3.45 gears?
My speedometer says SN6409/08  1120.<

I don't remember what model car you have.  I presume TR6 from the CDnnnnn
differential.  I do not remember if the diff of the TR6's had CD, but I guess
they did.  Measuring the wheel and engine turns is the most reliable method.
I have a hard time being sure about anything less than 1/4 revolution, so
I can just barely be SURE of the difference between 4.1 and 3.7.  It certainly
SEEMS as if you have a lower ratio than 3.7.  I am not sure of the options
available for the TR6, but I believe there were lower ratios than the 3.7.

Your calibration of 1120 says that you have 1120 speedo cable turns per mile.
The speedo drive is 1/2.5 of the drive shaft rpm.  So you have 2800 drive shaft
turns per mile.  Since I doubt that you have SMALLER tires than the TR4 series,
then your differential HAS to be a lower ratio than 3.7.  It would appear to
be, indeed around 3.5!

I assume that others more knowledgable than me will chime in here!

-Tony

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