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TR2/3/3A Tr3: Change in Diff=Change in Speedo reading?

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karls59tr

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I have the diff from an OD Tr3 in my car which is non OD.I think that is 3.7:1 to 4.11:1 or vice versa. I understand that at 6000rpm the speedo should read 60mph. Mine reads 5000or 5500 at rpm at 60.Is this reading because of the diff change? I also have a TR6 trans in the car. Would this make a difference?
I'd like to see the needles on both straight up at 60mph.....maybe I can gingerly relocate the tach needle?
 
TR3driver

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Only some OD cars had the 4.11 gears originally. From what I have seen, the majority still had 3.7.

But of course the gears were sometimes changed, even by dealers, so there are no guarantees.

The actual speed/rpm relationship varies with the tires you have fitted, of course. But the original spec for 3.7 + OD was 2520 rpm @ 60 mph (with radial tires). 4.11 + OD gives 2700 rpm @ 60 mph.

If you are really seeing 5000 rpm @ 60 mph, you have either found an axle with a much shorter ratio or your tach is wildly inaccurate. Either one is possible.
 
TR3driver

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Just for clarity, the article linked above was written by Tony Rhodes. TeriAnn just reposted it on her web site. You can find an updated version at
https://home.comcast.net/~rhodes/speedo.pdf

Tony mentioned some time ago that he was looking for a speedo from a TR3/4 with 4.11 gears (hence a calibration number up around 1320 or so). I don't know if he is still looking, but if you have one you'd be willing to part with, you might ask him.
 
Geo Hahn

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karls59tr said:
...I'd like to see the needles on both straight up at 60mph.....maybe I can gingerly relocate the tach needle?
Click to expand...

Yes. Tony's article explains how to carefully hold the drag cup while changing the position of the needle. Works as a final calibration for the instruments. Sort of works as the only calibration though you may find that the correction is non-linear, i.e. the speedo will only be accurate over a portion of its range. Still, that is often enough to know your speed in the range where it matters most and avoid a ticket.

But, in my experience speedos are more prone to error than tachs. In any case I would try to calibrate both for best result.
 
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