Speedometer Ratios

[James AlfordParticipant @jamesalford67616]

I have the attached speedometer which I wish to use on an Austin Seven. I do not know what vehicle it is from.

The Austin speedometer does 1040 turns per mile, but I am not sure what this new speedometer's turns per mile are. I suspect that it is 800, but I am not sure. Can anyone clarify what the small numbers near the oddometer mean? I can then work out what gearing I would need to put into a small ratio adapter gearbox or perhaps print a new dial face.

For clarity, the numbers are SN3253/07 and 800. It is 3" od if that helps at all.

Thank you.

James.

 

Edited By James Alford on 05/03/2020 07:28:21

Edited By James Alford on 05/03/2020 07:28:38

[James AlfordParticipant @jamesalford67616]

[RedsetterParticipant @redsetter]

800 is 800 turns per mile, the SN is a Smiths part number.

[Robert Atkinson 2Participant @robertatkinson2]

Calibration of this type of speedometer depends on the action of rotating magnets on a non-ferrous disk (eddy current torque) The maget strength and distance to disc are critical. If the distance between magnet and disk is fixed the calibration can be changed by de-magnetising the magnet. Age may have already moved the calibration in the right direction. Changing this will not correct the odometer as it is direct drive.

Robert G8RPI.

[not done it yetParticipant @notdoneityet]

It will be 800 tpm as stated above. Easily checked by driving it with a fixed speed drill for a measured time and noting the difference in odometer reading? Alternatively, driving it at 800rpm in the forwards direction should give an indication of 60mph, so can be checked at any other input speeds which are within the range of the instrument.

[John MCParticipant @johnmc39344]

That looks like a chronometric rather than a magnetic instrument, could be wrong. Have a look at Gaggs speedo website, might help.

John

[Clive Brown 1Participant @clivebrown1]

My memory of chronometric instruments on 'bikes is that they are identified on the dial, but could be wrong. If it's driven the needle will move in small steps rather than smoothly.

[Former MemberParticipant @formermember19781]

[This posting has been removed]

[Nick Clarke 3Participant @nickclarke3]

Many years ago I was involved in fitting the first mechanical tachos to trucks and we had a selection of splitter gearboxes that went into the speedo cable to provide a second cable to drive the tacho.

The gears in these boxes were chosen from a large range that Lucas provided to give the correct ratio. Still a stock item?? Probably not after all this time, but it might be worth a web search to see perhaps?? Or maybe talk to a speedo repair business?

Fitting a gearbox with one cable left off might give you the drive in the form you want.

[Bob Brown 1Participant @bobbrown1]

It might be worth speaking to Speedy cables as they can recalibrate speedometers **LINK**

[Richard MarksParticipant @richardmarks80868]

Calibration is 800rpm to show 60mph, speedo should indicate 60mph at 800 revs cable speed.

[Neil WyattModerator @neilwyatt]

An app like google maps or a GPS will give you the easiest way to calibrate the speedo.

You can also use it to do a journey of known length to check the odo.

Neil

[Richard MarksParticipant @richardmarks80868]

Odo and speed indication will be matched by internal gearing, so if you drive the instrument at 800 revs it will show 60mph and one mile will be added to the odo in one minute.

[Neil WyattModerator @neilwyatt]

Posted by Richard Marks on 05/03/2020 12:41:30:

Odo and speed indication will be matched by internal gearing, so if you drive the instrument at 800 revs it will show 60mph and one mile will be added to the odo in one minute.

The speedo function has to integrate rotations per unit time so it is usually driven by a magnet like an old electricity meter. This means the speedo reading can vary with calibration while the dodometer ration is fixed.

Neil

 

It's worth remembering that speedo and odo accuracy can be no more accurate than the known tyre diameter. This can vary by 1-2% just from legal tyre wear, let alone variations due to different types of tyre and state of inflation…

A good summary here:

http://www.wewantanycar.com/news/car-speedo-how-fast-are-you-going/

Edited By Neil Wyatt on 05/03/2020 14:17:36

Edited By Neil Wyatt on 05/03/2020 14:20:35

[Richard MarksParticipant @richardmarks80868]

Many years spent as a Speedometer engineer and Calibrator of all models and Police instruments in the 60s means I should know a little bit about them, as I said before run it at 800 rpm and note the results to see if it is accurate, if not send it to a calbration company to get it checked.

[John Paton 1Participant @johnpaton1]

Faced with a similar dilemma, but recognising the probable need to calibrate several times with different dis tyres and different ratio differential I elected to use Hall effect sensor on the driveshaft and a digital to analogue converter ( drives a motor which then drives the speedo!)

you can then set it to whatever conversion ratio you require.

Not cheap but the easy solution if you may need various ratios and prefer an original anologue speedometer clock.

[NimbleParticipant @nimble]

Hi James,

You could check out http://www.howardinstruments.com.au site as they have a number of ratio change setups, but I have no doubt there are many UK vintage car speedo specialists that would be able to assist you.

Regards, Neil

[James AlfordParticipant @jamesalford67616]

Thank you for all of the suggestions and information, which will be a great help. Knowing that the speedometer is 800 tpm is useful. I shall have a look at the various websites which have been suggested and see whether I can find a device with a known number of revolutions per minute with which to test it.

James.

[not done it yetParticipant @notdoneityet]

I believe Declan Burns (to be found on the morris minor owners club forum can actually supply a modified drive to convert different speedos, when modifications are made to the car.

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. Calibration of police devices have to be certificated, of course.

[HopperParticipant @hopper]

Posted by not done it yet on 05/03/2020 22:36:05:

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. Calibration of police devices have to be certificated, of course.

Not just the old mechanical speedos. Most modern electronic speedos are set up the same. They generally read up to 10 per cent optimistic. I guess manufacturers don't want their drivers to have a bad impression of their car by getting too many speeding tickets.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 05/03/2020 22:36:05:

[…]

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. …

.

Which is [approximately] +/- 5% of the actual target reading … it’s just that they shift the target to accommodate that ‘zero allowance for pessimism’

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Neil Wyatt on 05/03/2020 14:15:02:

.

… let alone variations due to different types of tyre and state of inflation…

.

That brings back memories of a protracted debate in the ‘Reliant Sabre and Scimitar Owners Club’

… where it was concluded that [within reasonable limits] state of tyre inflation makes no difference to the speedo accuracy; because the circumference of the tyre is what matters, not the rolling radius.

MichaelG.

[not done it yetParticipant @notdoneityet]

Posted by Michael Gilligan on 05/03/2020 23:28:38:

Posted by not done it yet on 05/03/2020 22:36:05:

[…]

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. …

.

Which is [approximately] +/- 5% of the actual target reading … it’s just that they shift the target to accommodate that ‘zero allowance for pessimism’

MichaelG.

No. Not plus or minus 5% at all. It is to prevent owners inadvertently speeding. A simple legal requirement that speedos provided must not indicate a lower speed than the actual speed.

[Mike PooleParticipant @mikepoole82104]

A tuned speedo is much cheaper than hot cams and gas flowing etc.

Mike

[Michael GilliganParticipant @michaelgilligan61133]

Posted by not done it yet on 06/03/2020 08:34:39:

Posted by Michael Gilligan on 05/03/2020 23:28:38:

Posted by not done it yet on 05/03/2020 22:36:05:

[…]

Incidentally, mechanical speedos are notoriously optimistic on speed indication. The legal range is to indicate up to +10% optimistic but with zero allowance for pessimism of the true speed. …

.

Which is [approximately] +/- 5% of the actual target reading … it’s just that they shift the target to accommodate that ‘zero allowance for pessimism’

MichaelG.

No. Not plus or minus 5% at all. It is to prevent owners inadvertently speeding. A simple legal requirement that speedos provided must not indicate a lower speed than the actual speed.

.

I am perfectly aware of the logic, thank you

… It’s the practical application of statistical quality control that I was trying to discuss.

MichaelG.

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