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TR2/3/3A Overdrive speedometer

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carpecursusII

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I have a TR3 that I added overdrive to. I used the original speedometer and it seems to be reading very high. Is this a gauge calibration problem or do I need a different gauge. The calibration number is 1200 on this gauge.
 
TR3s used many different speedometers, depending on original final drive ratio and tires. We had a thread just a few days ago on the variations. 1200 I believe would be a later car, with 3.7 axle and Michelin-X tires.

My guess is that you've gotten a non-TR OD, with a different speedo drive ratio. All TRs used a 5:2 ratio in the gearbox, up to mid TR6 anyway (I forget the change point offhand, same time the J-type OD was introduced), but other cars (eg Volvo) used the same OD but with a different ratio. How accurate are the trip meter and odometer?
 
carpecursusII said:
The gearbox is TR6 so it is possible I have the wrong ratio. Anything I can do to help it?
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Basically 3 choices:
1) Change the drive gears in the OD
2) Find a speedo with the appropriate calibration (there are shops that can rebuild yours with a different calibration or put a GPS movement in it).
3) Source a ratio adapter (and additional cable) to go between the gearbox & speedo. Last time I looked into this, a local shop quoted me $100 for the box, gears and short cable. That was a few years ago tho, so the price may have gone up.

But a TR6 wouldn't have the wrong ratio, unless you got a J-type OD. And the gears in the J-type would make the speedo read low by about 20%, not high.
 
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