Speedometer Ratios

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

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

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.

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.

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

John

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.

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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.

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

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

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

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:

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

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.

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.

Hi James,

You could check out 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

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.

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.

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.

.

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.