Tach and speedo inaccuracy. How to calibrate.

[MickG]

Hi John,
If you are going 54 MPH and the speedo shows 70 MPH yes it is wrong. But only in as much as the indicated speed shown is not corresponding to the actual speed.
If the engine is revving at 4000 revs the output from the gearbox is also at that speed in top gear. The internal drive gear in the gearbox drives the speedo and shows 70 MPH. It has no way of knowing what speed the rear wheels are rotating only the speed of the output shaft
In my previous post I made a few assumptions and having looked in my plus2 manual the speeds per thousand revs are not shown they are as follows:-
3.9 final drive = 16.5 MPH per 1000 revs in top
3.77 final drive = 17.4 MPH 1000 revs in top
These differ slightly dependant on tyres.
Now your rev counter shows 4000 revs = 70 MPH and 5000 shows 90 MPH which corresponds closely to the above using a 3.77 dif ratio.
The gearbox doesn?t know what ratio you are using and you have to change the speedo drive gear to correct this.
IMHO the rev counter and speedo are correct and the axel ratio is not one of the ratio?s normally used.
I could be wrong and probably are but the rev counter and speedo seem to be giving the correct read outs. Try this go for a ride and keep at about 3000 revs in top the speedo should read 52 MPH. You can make your own speed per 1000 rpm calcs from the above info and compare.

Regards MickG

[Johnfm]

Well. - 2 things.

I made a typo. It was indicating 80 and a bit when actual speed was 54.

And the odometer is reading 1.6 times actual.

I wonder if the 1.6 factor is a coincidence if whether the speedo or drive gear have been tinkered with to indicate km/hr on the speedo.

Will now jack the rear and spin the rear wheel one rev and count the diff revs.

[MickG]

The plot definitely thickens. Just remember anything is possible.

Judging by what you have just revealed you have the only 160MPH plus elan on the road or indeed anywhere.
Don't worry about the true speed you've got good bragging rights

Regards MickG

[Johnfm]

It is bloody annoying. Why would anyone do that when there is km/hr marked on the gauge?

Next step - remove and open speedo to check innards.

[ricarbo]

John,
I notice Maplin are selling a digital automotive test meter with tacho function and dwell meter and an inductive couplingm for ?24.99. More likely to be accurate than vintage equipment off ebay, at a reasonable 2%. Not only could you very easily and quickly nail your tacho with this, but it would be handy checking your distributor if you still use points - if the correct gap doesn't give the correct dwell, then the cam is worn and if the dwell reading is unsteady the shaft or bearing is shot.
I hate to say this, but my belief is that you can't tune your speedo by adjusting the gap between disc and magnets, as there is no provision for this in the instrument. Miniature gearboxes are made, or so I've heard, to alter the speed of the speedo cable, but I've no idea where you could get one. I may be mistaken, but I have never seen a report of any successful DIY speedo recalibration.
regards
Richard

[rcraven]

Speedograph sell various correctional gearboxes. May be worth a look: http://www.speedograph-richfield.com/html/minature_gearbox.html

Ebay and Amazon have cheap (about ?7 on ebay) little digital tachometers which may (or may not) be sufficient to check your calibration.

[Galwaylotus]

Why not just send it to Speedy Cables for a professional rebuild and recalibration? I had my tach done and converted for neg earth and electronic ignition. Very pleased with the result and good value for the money in my opinion. I'll probably get the speedo done as well but I wanted to wait until I got my new tyres as the rear sizes are different.

[billwill]

Detailed instructions for dismantling & cleaning a speedo are here:

http://magnetic-speedometer-repair.com/index.html

Very interesting for seeing how difficult it is.

It rather glosses over the issue of recalibration probably because they offer to do it for you for a fee.

They talk of remagnetising the magnet as part of calibration.

Personally I always thought that fine calibration was done by adjusting the point along the hairspring at which it was pinned to the fixed frame, but I have not looked inside one of these for many years, so that is just a vague memory. The diagrams on that website are not precise enough for me to see that particular detail.



~~~~~~

The Speedy Cables website describes how to measure your real rate of rotations per mile of your speedo cable, so that they can calibrate it dead accurately. It involves marking your rear wheel and then rolling the car along a road for 6 revolutions of the rear wheel and counting the revs of the dashboard end of the speedo cable.


http://www.speedycables.com/calibration.html

[billwill]

Hmmm, Interesting.

On a web page dealing with a speedo calibration instrument it seems clear that calibration is done by varying the strength of the magnet.

http://www.edn.com/electronics-news/438 ... eedometers

Analog speedometers use no electrical components during operation, but they employ a permanent magnet.

The calibration jig includes a demagnetizing coil that the system positions close to the permanent magnet in the speedometer. During calibration, our system passes a controlled AC current through the coil to slightly demagnetize the permanent magnet.

The magnet is purposely manufactured in an over-magnetized state so it can be demagnetized to the right level during calibration. But during calibration, the system can only demagnetize the magnet. It cannot re-magnetize the magnet.

Therefore, before calibration, the meter?s needle will indicate a higher-than-expected speed for a specific rate of rotation on the speedometer?s input shaft. For example, when a speedometer?s shaft rotates at 400 rpm, it should indicate 60 mph.

Instead, it points to about 100 mph. The calibration system applies demagnetizing pulses to the meter so the needle will drop to 90 mph, then to 80 mph, and so on until the meter indicates 60 ?1 mph. Then, the calibration system turns off the pulses.

[stugilmour]

Interesting info Bill. I sent mine off as well and it came back great (different company in the US). I took the view "No user servicable parts in here, keep your mitts off". If I was going to fiddle with a Smiths speedometer internals, I would definitely buy a cheaper spare off eBay to wreck first.

[Johnfm]

Furtehr 'testing' against satnav and actual road speeds show the speedo to be more and more over reading as speed increases.

At 20 mph, Speedo reads about 25.

At 60 mph, the speedo reads nearer 90.

At a 'true' 60 mph, given my tyre size of 165/70/R13 my engine should be running at around 3480 rpm - but rev meter was indicating closer to 4500 rpm.

So, looks like both gauges off for re-calibration.