I'm posting this because I spent several hours searching and for some reason this simple info is a closely guarded secret and very hard to find.

The speedos in C3's have a spinning magnet inside an aluminum cup and a spiral return spring. The needle is attached to the cup. As the magnet rotates it drags the cup with it against the return spring and this creates a speed reading.

The magnet is not a permanent magnet but rather just steel that has been magnetized. Due to this it will slowly get weaker and after 40+ years will almost certainly read low.

I had bought a new repro speedo and threw my original away. Big mistake as the repros are low quality. Luckily a guy local to me has a small business with C3s and had an original in stock. I used some parts from the repro and made the original functional and pretty.

However it suffered the weak magnet problem. Going 40 mph it only read 32 and the error increased with speed.

The return spring is adjustable and it would be easy to think that it just needed the spring adjusted. This is a mistake though as the spring adjustment is a fixed offset. What I mean is that if the spring is adjusted looser, for instance, it will read a certain mph more at all speeds. And if it is adjusted more than 2-3 mph the needle no longer returns to zero.

So the real fix is to re-magnetize it. But how? Yeah, I could always send it to a speedo rebuilder and hope they get it right but I'd much rather DIY.

Turns out it is really easy. The speedo has to be disassembled completely until the magnet is exposed. It's shaped as a flat bar with the ends curved up making it 'U' shaped.

First tool needed it a compass. I got a $7 one from Target. This let me identify the north and south poles of the magnet.

Next is the magnetizer which is just a stack a neodymium magnets with common 1/8 flat stock at either end. I used the compass again to identify north and south of my tool.

To use it, the ends of the tool (bottom in pic) are rubbed against the top of the 'U' of the speedo magnet (The distance between the end-plates of the tool should mach the speedo magnet).

Important: The north pole of the tool goes on the south pole of the speedo magnet and vice-versa

I rubbed it around for 20 seconds or so and re-assembled the speedo. I left the return spring adjustment at the factory setting.

Just returned from a test drive and the speedo reads perfectly. Sometimes it's 1 mph higher than the GPS, sometimes the same. Can't get better than that!

The reason this works so well is because the factory magnetized the speedos until saturation. Because of this you just needs silly strong magnets and it will once again max out the speedo magnet. Seems you can't over-magnetize them.

The magnets were $8 at home depot plus the $7 for the compass for a total of $15. Sure beats shipping it off to a rebuild shop.

So here's the tool with key for size reference (I blacked out the cut as I saw on another forum where a picture of a key was used to make a copy and take the car):

 

 

This is a great example of problem solving. Well done.

 

Much Thanks!!! This is super information.

I had a 68 speedometer that I was rebuilding and came to a dead end when I found out you had to have special equipment to recalibrate it.

So, I just bought another 68 speedo and put this one back for future parts if ever needed.

Now I can probably get it working without sending to a gauge rebuilder.

So thanks again! Will print this out and save.

 

Thanks for the write-up. A tach works the same way, but registers twice cable speed. How do I adjust for that? Any ideas?

 

Haven't done anything with a mechanical tach as I've switched over to electric, but I wouldn't be surprised if the magnetizing works the same way as the speedo.

 

This is excellent info! I like DIY outside the box thinking.

My speedo works fine, but when the time comes I'll remember this.