PDA

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



karls59tr
07-03-2009, 09:28 PM
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?

GilsTR
07-03-2009, 09:31 PM
I think you will find the OD diff is a 4:11.
At least that is what my 3A is. Gil

GilsTR
07-03-2009, 09:42 PM
The following link may be of help to you.
Sorry it did not get posted on above post! Gil

https://www.tjwakeman.net/TR/PDFs/speedo.pdf

TR3driver
07-03-2009, 11:21 PM
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.

karls59tr
07-04-2009, 06:18 AM
Oops! I meant to say that I thought 3000 rpm @ 60 mph was the norm and mine was running 2500 or so @ 60.

karls59tr
07-04-2009, 06:41 AM
Thanks for the link Gil. Looks like Teri Ann was very thorough in her research.

TR3driver
07-04-2009, 09:46 AM
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
07-04-2009, 02:07 PM
...I'd like to see the needles on both straight up at 60mph.....maybe I can gingerly relocate the tach needle?

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.