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

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!

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?

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!

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?

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"

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!

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!

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

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.