S type speedo calibration

[robbo911]

In need of some help. I've had my speedo reconditioned but it appears that it is over-reading significantly. Whilst I can get it calibrated accurately, I am puzzled why it is out by 10%+. The speedo has the following reference on it: SN6326/26 1080. The car is a manual o/d 3.4 S type with the standard 3.77:1 diff and (I believe) standard sized 185/80/15 tyres. I was wondering whether someone has fitted a speedo from an auto (which I believe has a 3.54:1 diff) at some point in the past, or maybe there is some other explanation.

Thanks!

[piman]

Hello Robbo,

the last number (1080) are the turns per mile of your speedo. If you can find the circumference of the tyres fitted a calculation can be made of the final drive ratio to see if you have the correct speedometer?

Alec

[cafcpete]

Looking on a tyre size calculator site
https://www.tyremen.co.uk/tyre-size-calculator
A 185/80 R15 needs 757.1 revolutions to do one mile!
Make of that what you will!

[robbo911]

Thanks both. I did the calculation and came to the 757 TPM answer as well. When compared to the 1080 TPM on the speedo that is a massive difference and therefore doesn't seem to make much sense. Does anyone have a manual S type (3.8 o/d and 3.4 o/d have the same diff ratio as standard) and can post the TPM on their speedo?
Thanks

[piman]

Hello Robbo,

I agree that figure doesn't make sense.

However, it comes back to me how to check the speedo matches your car. It entails disconnecting the cable at the speedo end and making up a little cardboard pointer and attach it to the cable. Then mark the tyre at ground level, and use a marker on the ground opposite that mark on the tyre, then move the car one full turn of the pointer and measure the distance travelled. Divide that distance into one mile and the answer should be close to the speedo figure if it is right.

Alec

[B87UL]

The speedo number quoted ending 1080 is correct for a 3.4 ltr MOD S type on standard tyres, with the 3.77:1 axle. I have/ had that exact specification car but only this week have I re-geared the axle to 3.31:1 so I am familiar with the speedo variations.

This turns per mile relates to the instrument not the tyre revolutions.

Perhaps someone has rebuilt this speedo in the past with parts for the 3.54 axle ratio, but even then the readings would not be well over 10% out.

Remember when that car was built there was a 10% allowance in law for variation in the speedo reading.

[Zennan101]

Just curious. Does anyone know if there is any legislation regarding the accuracy of speedometers and presumably changing anything that alters the speedo calibration is a notifiable modification as far as insurance is concerned?

Rod

[piman]

Hello Rod,

from memory I believe + or - 10%? I don't know about insurance companies attitude?

Alec

[Zennan101]

Hi Alec,

Nothing to do today but hide from storm Dennis so plenty of time to search the internet. It would seem that speedometers are permitted to read up to 10% over true speed but there is no leeway to read under true speed.
It also seems that insurance companies view any changes to transmission as notifiable modification.

Interesting trivia!

Rod

[robbo911]

Thanks Everyone.

When the weather gets a bit better I will need to do the exercise with the cardboard pointers. Hopefully that will reveal all...

Will post again once I have the results

Cheers

[robbo911]

Thought I'd update you all. Cardboard pointer exercise suggested that the problem was not the speedo. By process of elimination, it was something at the gearbox end, which was a bit curious as I was under the impression that all the drives at that end were the same. Not so, it appears. The E type used an angle drive which has a ratio of 1:1.27 - part number C25547, Smiths ref: BG2402-08. S types used C24589, Smiths ref: BG2402-7, with a 1:1 ratio. My angle drive unfortunately had the Smiths ref partly rubbed off, but it looked like it ends in 08. So I took a punt and ordered up a replacement 07 drive and installed it. Problem solved! I now have a surplus E type angle drive...

I just need to fix the oil pressure gauge/sender (now decided to read just >0 at all times), plus the million and one other things on the long list.

Thanks for all your help!