[TR] Overdrive and Sppedometer

[DAVID MASSEY]

 If Randall was still with us he could tell you when this happened.  I'm not sure when but I think it was right about this time.
In the olden days all gearboxes cam with the same speedometer drive gear and the speedometers were calibrated based on the rear axle ratio.  If you look closely you will see a small print number indicating the number of turns per mile.  It could be some number like 1123 or 1056.  

Then they adopted the more industry-standard 1000 turns per mile speedometer.  With this scheme they would fit different speedometer drive gears according to the rear axle ratio.  

You may be able to fix this with a different driven gear, which is easily interchangeable in situ from under the car.  You might check the archives.  There is probably a graph showing what color gear (they are color coded) you need for the axle you have.  OD or no OD is of no consequence since the gear is driven off of the transmission output shaft.  

Let us know what you find.
 
Dave 

 
 
-----Original Message-----
From: Jeff Scarbrough <fishplate at gmail.com>
To: Triumphs (triumphs at autox.team.net) <triumphs at autox.team.net>
Sent: Mon, Aug 24, 2020 8:13 am
Subject: [TR] Overdrive and Sppedometer

I have a 1975 TR6.   With the original gearbox, the speedometer was correct.  I took it out to add overdrive and put in a gearbox from a 1974 while I worked on it.  That gearbox made the speedo read about 12% higher.
Got the overdrive on the box and in the car.  My speedo still reads high.  I'd like to find a simple solution...
Do I try to find the right drive gear for the gearbox output, or do I get a different speedometer?

Jeff ScarbroughCorrosion Acres, Ga.** triumphs at autox.team.net **

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