Speedo Overreading

[ed2204]

Hi,

My speedo's been overreading and had a wobbly needle, so after reading some articles on here I've replaced the cable and it's stopped wobbling. However, it's still overreading - at 30mph on the speedo it's actually going 20mph (GPS), 40mph it's actually 30mph etc. up until 70mph, where it's actually going 55mph, and 80mph, where it's 60mph.

The numbers on the speedo are SN 4419/04 and 1376, and the mog's a standard 1967 1098 model.

Any ideas? Or do I need to get a new speedo/get the current one calibrated?

Ed

[Declan_Burns]

I think you may have the wrong speedo fitted. It should have 1408tpm and not 1376 assuming you have the standard diff. See photo attached just under the mileometer where it shows the speedo tpm.[frame][/frame]
Declan

[RobThomas]

Looks like you might have 3% difference in the 2 gauges but it also looks like your needle is not zeroed at the right position.

Can you see what the needles rests on at zero? Should be some sort of stop made from a piece of wire, or similar. If you temporarily push that wire in then the needle ought to go into a neagtive reading and come to rest abeam a small dot marked at somewhere near -5 or -10 mph. That is where you needle ought to be, so, if it isn't resting there with the wire away then you could lift the needle off the spindle and relocate it.

Just my suggeston. I'm sure someone eklse will be along soon with another idea.

Good luck.

PS Might be worth plotting actual mph against indicated mph to see if they go up evenly and if their figures meet at zero.

[bmcecosse]

As Declan points out - it's the wrong speedo - should indeed be 1408 for your car.

[ed2204]

Thanks for the replies - I'll get a 1408 speedo and try that.

[bmcecosse]

Certainly the difference between 1408 and 1376 is minuscule - and certainly doesn't explain the excessive readings you have. But you may as well start with the correct speedo and see how that goes. But you could certainly try re-positioning the needle on the speedo you have. Have you any feel for how accurate/inaccurate the mileage reading is on your speedo ?

[MarkyB]

How wobbly is it?
If you are taking the top reading as the actual one, then it will read high.

It sounds like the cable needs some attention, another speedo head won't fix it.

[bmcecosse]

The cable has already been replaced ! Possible the wild wobbling has moved the needle on the spindle........

[MarkyB]

Doh!

[ed2204]

I've tried a 1408tpm speedo and it's still overreading - however, 60mph road speed is now only 75mph on the speedo, compared to the before 80mph displayed. I've pushed the pin in on the 'new' speedo and the needle rests at the dot.

Is it a case of adjusting the needle to read the right speed, or should I try the flag on speedo cable, spin the wheel method to check the tpm of my car?

[bmcecosse]

You must have a 5.35 final drive ratio!

[fussyoldfart]

I have been shopping for a speedo for my '67 because the odometer has quit. Mine is US spec and the TPM is 1376 (4419-04).
I have noticed that speedos for export to other parts of the world with a KPH scale as well as MPH are TPM 1408 (4423-04).
The difference is not enough to matter at 2.3% but a huge difference such as you are seeing, Ed, might well be due to a changed axle ratio.

I think you need to start with a flag on the prop shaft and count the turns. Don't forget if you turn only one rear wheel it takes twice as many turns. If the diff ratio is right at 4.22 perhaps the gearbox has been swapped out for an earlier one; is it a smooth case or ribbed? I think the speedo drive in the older box is set up for a 4.55 diff.

Darrell McDonald

PS - If you want to sell one of those speedos PM me with the price.

[bmcecosse]

Gearbox speedo drives are all the same - assuming a Minor gearbox that is !

[ed2204]

Hi,

I've tried jacking the car up, turning one wheel and counting the number of times the propshaft turns, and I've got a range of results, from 3.875 to 4.88, all averaging out at 4.325. I've also counted the amount of times the speedo cable turns in relation to one of the wheels (The other still on the floor), and the cable turned 7 times for 8 revolutions of the wheel.

I've tried looking at the diff to see if I could see the part number, but the only markings I can see is 'MOWOG' stamped on the back.

Ed

[bmcecosse]

You can't get a 'range' of results - it's a fixed ratio - must get exactly the same result each time!

[fussyoldfart]

If something has been changed, like the diff ratio, you need a speedo that matches. Try this:
Follow someone in another car that has a known good instrument. Record your miles driven to the exact 1/10th over a distance of at least 25 miles (100 would be ideal but impractical). Divide your recorded miles by the precise actual distance from the other car to get a factor (ie 1.105 or whatever it turns out to be) and multiply the TPM on your speedo by that factor. The result will be the theoretically correct Speedo TPM required for your car. Then you can look to see if there is a speedo with a TPM that is near as possible to the number you get.

If the odometer is correct and the instrument is not faulty the speedometer will be as good as Smiths can give you.

It's late just now, time for my bed, so some other smart-ass may tell us that I have the arithmetic backward but I think this is easier than crawling under the car to count driveshaft revolutions.

Darrell

[bmcecosse]

The relationships are already well known - and the car you follow may not have accurate equipment anyway. If in doubt - use a GPS!