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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?
 
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.
 
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.
 
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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