Classic cars forum & vehicle restoration.

[rob needs]

Over the time I've had this car the speedo has registered the wrong speed.I have used about three spare clocks,another cable.I think its the gearbox end thats giving the problem,is this a real pain to do,can anything be done well the box is in the car.I dont fancy taking out the box.its just with all the cash cows about,sorry i mean speed camera's.i find myself going super slow through camera's to make sure

[Ashley]

Speedograph Richfield in Nottingham or Speedy Cables in Cardiff will overhaul and re-calibrate it for about £70. It will need it and it's worth it to avoid a speeding ticket.

In Northampton, which is a Police State and an excellent source of revenue for loveable Gordon and his delightful chums, I went through a speed camera in a forty limit with forty-one on the speedo and got fined and three points for doing forty-eight in the MKVI. Bloody annoying and expensive.

Ash

[Jim.Walker]

Two points you might consider:-

1. Are the correct section/profile tyres fitted, and wheel size?

2. Are the speedos you are using calibrated to the correct number of "turns per mile" (tpm)? Generally, speedo cables turn at somewhere between 800 and 1200 tpm. On Smiths and Jeager speedos (in very small digits) the tpm is marked at the end of other numbers on the speedo face. Speedos might look identical, but some might be intended for similar cars with different back axle ratios or wheels sizes etc. I am making an assumption that your speedo will be Smiths or Jeager however.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[MVPeters]

Just to add to Jim's accurate comments:

The very first thing to do is to find out how much your speedometer is 'off' - & the simplest way is to use a GPS for a few days. (Then maybe put dots on the speedo at the correct speeds?).

Then find out why the speedo is off. If you've tried 3 clocks with similar results, it's not the clock - so it's probably the tyre size or less likely the rear axle ratio,
I can tell you from recent experience that changing from 225/60x16 to 70-series tyres (same size, but snows & a different mfg) throws the speedo off by 2-3MPH at 30MPH to 4-5MPH at 70-ish - ie comparing the fairly accurate digital dash at an indicated 30MPH with the GPS, I'm actually doing 32-33 MPH. Enough for a ticket/no ticket difference.

A Jim says, there's a tiny number under the odometer on most Smith's/Jaeger speedos. Somewhere on the web you'll find a formula to calculate that number for your car - it will take some effort to make the measurements. (Ignore the US sites - apparently all US speedos run at 1000 & the adjustment is made by changing the drive-gear-box on the transmission - this may not be 100% accurate).
At one time Smith's had a reference manual that showed that tiny number for all their speedos; I found a MkII Jag speedo with a very similar number that was a perfect fit for my 850 Mini!

So the solution is to find a Sceptre speedometer with a different number. You might check to see if automatic or estate versions had different gearing & therefore a different speedometer. I can't imagine there were that many options of axle ratios on Sceptres, so the likelihood is that it's the tyre size.

Hope this helps.

Mike

[Jim.Walker]

MVPeters says you can find the turns per mile on the 'net.
I don't know about that, but faced with a similar problem where 15" wheels were fitted to a car which formerly had 13" I did the following:-

I found a bit of flat ground and marked off 176 feet (1/30th of a mile). I checked the tyre pressures, then I dropped a plumb line from the front bumper, just clear of the ground and lined the car up with the first mark. Then I disconnected the speedo cable from the speedo and attached a card "flag" to the inner cable so I could count the number of turns as it rotated. With the help of a friend we pushed the car to line the plumb bob up with the finishing line counting speedo cable turns as we went. All that is left is to multiply the cable turns by 30 to establish the turns per mile.

The difference was very significant and no matching speedo seemed to be available. However, I found a Smiths speedo on a Triumph 2000 within 2 turns of my target and found it easy to transfer the innards and dial to my unit so it matched the other instruments. The result seems to correspond correctly on checks against other cars and over known distances. I've never tried a GPS test and I'm not sure they are accurate enough.

Of course any reasonable distance can be used with a little simple maths.

Jim.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[rob needs]

Thanks for replies,i did check those numbers at the time on the bottom of the clocks.I checked them because one of the cars that some of my spares came from was a automatic sceptre.
I'm talking of about 20mph out on the clocks mind! I like the idea of marking the clock face.Maybe i should get someone driving in front of me doing 30 then 40.

[victor 101]

[Jim.Walker]

I tried a Sat Nav while travelling down the Grand Union Canal near Watford Gap Services to try to check the boat speed. It decided I was stationary in the Southbound Lane of the M1 2 or 3 hundred yards away. And that was in pedestrian mode because I thought it would zero in on the towing path as a public footpath about 15 foot away!

Apart from that did I not read somewhere that for commercial uses the system is only accurate to within 15 ft? That could have a significant effect on speed readout. I don't doubt your experience, but can it always be relied upon?

Jim.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[MVPeters]

These links will tell you just about everything you never needed to know about speedometers - & there are plenty more.................

http://triumph.daveola.com/NOTES/Speedo_Repair.pdf - especially page 18 & the three appendices.

http://www.westnet.com/~mfrank/Speedometer/Smiths.html

http://www.tomsminisite.co.uk/speedo_calculator.php


This is how to calculate the speedometer 'code' for a Smith's/Jeager speedometer:

Jack up both driven wheels, measure the circumference of the tyre in inches. = "A"
Remove the speedo drive cable and fix a pointer (cardboard etc) to the end
of the cable.
Turn driven wheels forward ten turns ( use chalk mark to confirm)
Ensure both driven wheel turn a full ten turns. Alternatively turn one
wheel a full 20 turns with the other wheel not turning at all.
Count the turns of the speedo cable to the nearest qtr turn. = "B"

Calculation is then as follows:
(1760 X 36) = inches per mile. divided by wheel circumference "A"= wheel
turns / mile "C". Multiply by the speedo cable turns "B" and divide by 10
Typical example:
"A" = 80 , "B" = 12.5
1760 X 36 = 63360 / mile divided by "A" (80) = "C" (792 Wheel turns per
mile) X by "B" (12.5) = 9900 divide by 10 = 990 = new speedo calibration number.

or 63360/A*B/10

From: http://www.bighealey.ltd.uk








Mike
____________________
MVPeters@comcast.net

[MVPeters]

I posted the speedometer calculation formula as a separate post in case anyone wants to keep it for reference. There is another, that I can't find, that calculates the same thing based on knowing the rear axle ratio & the speedo gear box ratio - the only thing to measure is the tyre circumference.

There's something going on with this Sceptre!

You've changed the speedo heads & the cable - were the results all essentially similar?
Is the needle stable, although at an incorrect speed?
If the tyres were the problem they'd have to be the size of a go-kart or an 18-wheeler to make a 20MPH difference.
Even an axle ratio difference (manual/automatic/estate etc) would only be off by 2 or 3MPH.

I can only think that the speedometer drive gear on the gearbox is missing some teeth - the cable still spins, but intermittently, resulting in a lower-than-expected speedo reading - & a wavering needle? These little gear boxes are usually a simple bolt-on unit & shouldn't take long to change.

Mike