9 replies to this topic

[Tornado99]

My car has a mis-matched speedo drive & speedo gauge. The gauge is reading some 10-12% too low compared to GPS readout. I know there are ways to figure out correct drive gearing and swap in the proper drive gear, with a fair amount of effort, but what about doing something like this guy has done to reposition the gauge needle to achieve pretty close accuracy:

 

 

I guess the odo will still be running too slow, but for a car with non-original speedo, unknown true mileage, does not really make any difference to me. 

 

 

 

[nicklouse]

moving the needle will change nothing except for at one particular speed.

 

just get the right parts in the car.

 

some reading

http://www.guess-wor...Tech/speedo.htm

 

[Ethel]

The odometer is what you need to be going off to judge the accuracy first.

[Tornado99]

The odometer is what you need to be going off to judge the accuracy first.

The Odo and needle movement are usually operated by different mechanisms. Usually mechanical speedo needles move against a coiled spring, forced by magnetic fields from a rotating disk attached physically to the speedo cable. The video I linked described this system better. Odo is just driven by direct gearing off the cable rotation.So it is possible to have needle movement not sync'd/calibrated to odo. 

Now it may be the Nippon speedo I have on the car now is not as described and uses a different movement mechanism.Never seen one apart to know for sure.

[Ethel]

True, but there's only one speedo cable. The odometer rigidly counts how many revs it does, which requires the gearing to provide the correct turns to distance ratio.

[bitsilly]

You can get a unit with a GPS input, which converts it to a mechanical speedo cable output (or some can use a VSS if you can set one up.

They seems super easy to calibrate and may end up cheaper than a rebuild.

 

https://www.speedhut...peed-Converter-(Cable-Included)

 

https://www.classica...ons.com/classic

 

https://www.ebay.co....353.m1438.l2649

[Tornado99]

True, but there's only one speedo cable. The odometer rigidly counts how many revs it does, which requires the gearing to provide the correct turns to distance ratio.

Understood. But I'm more concerned with false road speed due to danger of getting tickets at this time. If the ODO remains inaccurate, I can live with that. 

 

The GPS to mechanical cable gizmos are pretty neat. But at over $350 USD, I'd likely just go for a Smiths electronic speedo and use an inductive pick up for similar money. There are also Chinese made GPS speedo units for about $75 USD. 

 

I looked at the GuessWorks tool for calibration and gearing, but I really have no idea what to be punching into the calculator selections for my car, other than tire size. . 

[Ethel]

Simplest solution is to just stick markers to the glass or bezel of the speedo. As Nick says, it can only be right once if the needle and dial plot 2 different arcs.

 

That or learn the what it reads at common speed limits, or rely on a gps or phone app.

 

If you were to go to the trouble of taking it apart to get at the needle you could print yourself an new scale to stick on.

[bitsilly]

That sounds a good idea!

[Spider]

 

The odometer is what you need to be going off to judge the accuracy first.

The Odo and needle movement are usually operated by different mechanisms. Usually mechanical speedo needles move against a coiled spring, forced by magnetic fields from a rotating disk attached physically to the speedo cable. The video I linked described this system better. Odo is just driven by direct gearing off the cable rotation.So it is possible to have needle movement not sync'd/calibrated to odo. 

Now it may be the Nippon speedo I have on the car now is not as described and uses a different movement mechanism.Never seen one apart to know for sure.

 

 

Sorry, I haven't looked at the video above (15 mins is a little more than I feel I can sit through right now !) so I don't know if this is covered in the video.

 

First order of business here, I find, is as Ethal suggests, get the Odometer right by changing out the drive gears in the gearbox (if you can), This is important as it then puts the Speedo Head in the right ball Park from were it can usually be adjusted from.

I know on the Smith's unit (maybe on the Japanese units too), if you remove the Needle and the Face Plate, there's an Adjustment Screw that raises or lowers the Armature in relation to the Magnet that you mentioned further up. It's only a very small screw and doesn't need big movements to make adjustments, so go easy here. It only has a small range (about 10 to 15%), but unlike moving the Needle, this keeps it in Calibration across the whole scale.

Again, on the Smiths units, and I'm not sure on the Nippons, there are 2 very small dots on the face just down past (lower) than zero and the stop here. When refitting the Needle, it should be aligned between these marks, to provide the correct preload on the hair spring.

 

It's been a little while since I've done these, I moved on to an Electronic Speedo about 8 years ago and haven't looked back. Changing Tyres sizes or Final Drive Ratios, it's very easy and quick to just punch in some new numbers and off you go.