Speedometer gearing

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Chet
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Speedometer gearing

Post by Chet » Fri Jun 28, 2019 11:53 pm

Can someone please tell me the correct gearing (cable turns per mile) for an SE4a speedometer. I have two: a foreign one with 840tpm was in the car; I have another with the correct face and 960tpm which I think came out of an SE5.
Cheers,
John



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Speedometer gearing

Post by b.c.flat hat » Sat Jun 29, 2019 9:09 am

Ah, thought I'd seen it somewhere, in front of the GT workshop manual!
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Speedometer gearing

Post by philhoward » Sat Jun 29, 2019 10:23 am

Normally printed on the face of the speedo - I thought they were usually 1000 turns per mile for a Scimitar (GTE certainly) - maybe not for the Coupe?


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Speedometer gearing

Post by b.c.flat hat » Sat Jun 29, 2019 10:40 am

Hm, the gearbox speedo driven and drive gears are 21 teeth and 7 teeth so nearly the same as the diff (3.583 to 1), sort of equalising each other. I'm going to let you do the maths, not had enough coffee yet!☹️



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Speedometer gearing

Post by David Tew » Sat Jun 29, 2019 11:27 am

The original speedo in my SE4b was 960 TPM but my rebuilt one is 1,000 TPM.

If my maths is correct that's 4.2% lower geared which isn't too different, works OK with lower profile tyres and is within the original spec of +/- 10%. Also, the speedo usually reads high so a rise in TPM should make it more (but still not very) accurate.

Did Reliant change speedo drive gears depending on axle ratio?


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Speedometer gearing

Post by Roger Pennington » Sat Jun 29, 2019 12:09 pm

philhoward wrote:
Sat Jun 29, 2019 10:23 am
Normally printed on the face of the speedo - I thought they were usually 1000 turns per mile for a Scimitar (GTE certainly) - maybe not for the Coupe?
Yes my understanding had always been that they were all 1000tpm, but maybe that might have just been "all GTE" rather than "all Scimitar"


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Speedometer gearing

Post by GavinR » Sun Jun 30, 2019 10:46 pm

b.c.flat hat wrote:
Sat Jun 29, 2019 10:40 am
Hm, the gearbox speedo driven and drive gears are 21 teeth and 7 teeth so nearly the same as the diff (3.583 to 1), sort of equalising each other. I'm going to let you do the maths, not had enough coffee yet!☹️
So the speedo drive ratio is 3.0 and the axle is 3.583 which gives 837tpm (3 / 3.583 * 100) so the 840tpm speedo would seem to be a good choice!


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Speedometer gearing

Post by David Tew » Mon Jul 01, 2019 8:26 am

GavinR wrote:
Sun Jun 30, 2019 10:46 pm
b.c.flat hat wrote:
Sat Jun 29, 2019 10:40 am
Hm, the gearbox speedo driven and drive gears are 21 teeth and 7 teeth so nearly the same as the diff (3.583 to 1), sort of equalising each other. I'm going to let you do the maths, not had enough coffee yet!☹️
So the speedo drive ratio is 3.0 and the axle is 3.583 which gives 837tpm (3 / 3.583 * 100) so the 840tpm speedo would seem to be a good choice!
i'm not sure that's correct as you need to consider the rolling circumference of the tyre.

If we work backwards, 1 turn of the tyre gives 3.585 turns of the propshaft and 1 turn of the propshaft gives 1/3 turn of the speedo drive. Therefore the overall ratio is 3.585/3 which equals 1.195.

The rolling circumference of an original 165/82 x 15 tyre is quoted by Reliant at 830 revs per mile, although THIS SITE gives a figure of 786.6 (probably the true rolling circumference allowing for centrifugal force). If we take the average of 808 and multiply this by the overall ratio we get:
1.195 x 808 = 965.56 turns per mile.
Obviously this will vary on tyre/wheel size, axle ratio, speedo gears etc.

It makes your brain hurt! :wink:


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Speedometer gearing

Post by GavinR » Mon Jul 01, 2019 12:50 pm

Hi David,

Reading your explanation - yes I had forgotten the rolling radius of the tyre

I agree with your calculations so the 960tpm speedo must be the correct one

Thanks
Gavin


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Speedometer gearing

Post by Chet » Thu Jul 18, 2019 7:45 am

Thanks folks. Very helpful. My brain has now cooled sufficiently to be happy with 960.



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