Wildly inaccurate speedo

[jaguar68]

Hi chaps,

I've recently replaced the original 1098cc engine and 4 speed 'box in my '69 4 door minor with a 1275cc Ital engine and Ford 5 speed 'box due to the car jumping out of fourth. Other than a few frustrating electrical problems detail in another post, I'm more than pleased with the transplant.

The trouble is now the speedo is wildly inaccurate, reading anything from 5 to 20ish mph out. Using a speed app on the iPhone, the speedo reads 10 mph(ish) over at 30 mph increasing to 20-25 mph at an indicated 80mph, the phone reading a speed between 55 to 60 mph. There is also a slight "bounce" in the needle too, but the cable is new, coming with the gearbox kit and is running in nice easy loops without any kinks or tight bends.

I can accept that the speedo will be out after changing the 'box, but the indicated 80 mph was read whilst travelling along the A2 in Kent on the uphill section befrore Bluewater. I can't believe that the car is only really travelling at 60 odd mph as i was able to keep up with the general flow of traffic where the norm is 70 to 80 mph.

The car is running the standard 1098cc 4.22 diff at the moment and Bridgestone 145 R14 tyres.

Any suggestions?

Regards

John

[chesney]

If it over reads then its unlikely to be a problem with the cable.
Maybe the cable needs the correct pinion on the end?

[bmcecosse]

So - it's about 25% over reading - is that a fair bet? Compare it ACCURATELY using you phone as the guide - it will be dead accurate. And tell us the tpm marked on the speedo at the moment. I believe there are different gears for the speedo drive on the Ford box - but I am no expert on that box.

[jaguar68]

At 30 mph the speedo is reading 40 to 45 mph. at 60 the speedo read 80 to 85 mph, the error increasing as the speed increases.

The speedo in the car is a 1408 tpm, which I believe is the standard 1098 speedo. I'm aware that the speedo calibration will be out, but its a little disappointing to discover that the car is travelling much slower than expected. Unfortunately, I think it will be quite difficult, if not impossible to uncover what the speedo drive ratio in the Ford box is.

Regards

John

[bmcecosse]

You need to search for a 1000 tpm speedo - not sure if the Mini ever used that number - but worth a try. The reading won't be increasing with speed percentage wise...... possibly just a basic small offset and then a 30 % error throughout the range.

[jaguar68]

I was hoping to change the diff for a 3.9 ratio at some point, so this will affect the speedo calibration again. What effect will this have on the acceleration & top speed of the car?

Roy, what calculation did you use to calculate the required tpm for the speedo?

Regards

John

[chesney]

I seem to remember that someone had a similar problem and he had to go to an obscure shop to get the last pinion drive (apparently the right size is difficult to get hold of??)

[bmcecosse]

1408 is ~ 30% too fast - so 1000 should be near enough.......

[mike.perry]

1000 TPM is .a Series 2 speedo. The Series 2 has a different gearbox pinion ratio to the rest of the A Series boxes (2/9?), hence the ratio does not fit the normal pattern. Alternatively there is a 1276 TPM speedo

[daveyl]

Some MGB's, Marina's and, Triumphs have 1000 tpm but you would need a new cable (the cable attachment is different) and you will also have the problem of where to mount the new speedo, as well as what to do with the fuel gauge.

I would suggest it would be cheaper and easier (in the long run) to get your speedo recalibrated.

[charlie_morris_minor]

have you considered going down the electronic speedo route? with the type 9 gearbox it is straight forward and then when you swap your diff or change wheels / tyres it is a simple job to recalibrate your speedo.

it might be worth dropping jmcar a line as he was looking to go down this route a few months ago..

http://mmoc.org.uk/viewtopic.php?f=4&t= ... do#p435708

[C6Dave]

As already said, you could get your speedo re calibrated. JDO is the best I have found: http://www.jdo1.com/index.html

[bmcecosse]

Aye - but compared to a £5 speedo from ebay - it's a VERY expensive alternative.......

[C6Dave]

Aye - but compared to a £5 speedo from ebay - it's a VERY expensive alternative.......

Everything according to you is 'very expensive', but a recondition/recalibrated speedo will be accurate and better than a fixed penalty notice + points.............

[bmcecosse]

Of course I think it's expensive - I'm a poor old pensioner! My Speedo is dead accurate on both miles and speed - and was £5 from ebay....

[C6Dave]

Of course I think it's expensive - I'm a poor old pensioner!............

So am I

[jaguar68]

I don't want to go down the route of an electronic speedo, as the one linked to on Ebay look cr*p, cost more that I would like to spend and entail getting the electronic pickup etc, all adding to the cost. I'd like to keep the original style Smiths speedo as I would like to keep the dash looking mostly as it should.

I've found a1280tpm speedo I had lying about, but looking at the cable connection it looks like part o fthe square drive is broken. I'll try fitting it an see what happens.

Regards

John

[jaguar68]

Oh, looks as like the 1408tpm speedo has completely lost the plot. It has times of lucidity butthen the needle starts bouncing and runs round to the needle stop!!

[C6Dave]

Possibly contaminated with oil?
It can come from the gearbox via capillary action and put a small coating on the speedo interior parts

There is a post somewhere about stripping and cleaning a speedo head (if your really brave)

[jaguar68]

I'm not sure I'm brave enough to try to clean the speedo head myself, anyway the speedo will need to be recalibrated at some point due to the new gearbox. I'm wondering if 13 inch wheels would address part of the inaccuracy problem. The instructions with the 5 speed 'box say that the speedo will be right for a 3.9 diff & 13 inch wheels. Trouble is, I'm not sure that 13 inch wheels would fit over the Ford front disk brakes.