Speedometer off

[cyberdork]

So my speedometer is off 5mph at 0-50 mph, and of 10mph after 55. Any idea how to fix?

[tkent02]

Ride faster.

[Nessism]

Larger diameter tire helps correct the error somewhat. I'm not aware of any fix that can be made to the speedo internally.

[simon kuether]

Mine reads 120km when I'm doing 100km although the trip meter seems to read ok.Don't think there's much about it except be aware of it.Cheers,Simon.

[Matchless]

Apparently you can calibrate your speedo once you have it open by tweaking the spiral spring tension a bit, here is a guide: http://www.ibmwr.org/otech/speedo.html

I would add that testing it at a fixed speed with an speed controlled drill and noting the reading and then road testing it at the same speed to determine the actual error margin, say with a GPS, will make it easier. Thereafter use the drill once the gauge is open and set the tension to make up the error margin.
A table fork also makes it easier to remove the needle if needed.

Let us know if it worked.

[Nessism]

I took my 850 speedo apart because it oscillates back and forth somewhat randomly around the proper speed. The mechanism to drive the needle uses a magnet and metal drive disc of sorts. The needle itself is spring loaded with a damping mechanism of some sort to slow needle response. I think the problem with mechanism accuracy relates to 1) spring tension mentioned in that BMW article linked by Matchless, and 2) the damping mechanism. Tried to clean out the mechanism on mine, down where the cable comes in, using spray cleaner but the needle oscillation problem continues - although slightly less. Interestingly, the speed is now reading slower. Must have something to do with the damping mechanism but not exactly sure. The mechanism itself is staked together so it's not easy to take apart. I'd like to find an old mechanism to split open before doing surgery on my actual speedo. I'm sure it can be done but it will require some careful work.

[Matchless]

Here is a generic description of a speedo http://www.chickjunk.com/how-does-th...motorcar-work/
Basically if your speedo is jumping around and the cable is good as well as the drive on both sides, thus having the magnet spinning inside evenly, then the problem could be a sticking axle on the needle/hub bush. This could be old and dried out lubrication, no lubrication, corrosion or something bent or worn there or some dirt.
To get to the top of the needle bush, lift the needle carefully back over the zero stop pin and mark where it settles. Then put a piece of paper on the face and use a small table fork with prongs on both side of the needle base and prise it off. It is a press fit and must be pulled squarely in line with the axle to get it off.
Now unscrew the faceplate and you should have access to the needle bush. Soak it in a solvent that will not damage plastics, such as benzine to dissolve the old lubricant. Dry properly and relubricate with 3 in 1 oil. Check that it moves freely throughout the range.
Replace faceplate and push needle on at marked spot and lift back over stop pin to zero and test it with your electric drill and Bob's your aunty!