The speedometer in my 1971 MGB was reading quite a bit below my actual speed. I'd worked out a lot of the other bugs in the car but this was becoming a major irritation. There wasn't anything mechanically wrong, it just seemed that the rotating magnet had lost some strength over the years.

I expect most of you have looked inside a common speedometer at some time. There's a bar magnet, driven by the rotating cable, that spins in an aluminum cup connected to the pointer indicator. The magnetic fields set up a drag force on the cup that increases as the spin rate increases. If the magnet gets weak the indication gets to read low.

Needless to say, my amateurish attempts to remagnetize the motor merely resulted in demagnetizing it further-at one point it would read no more than 35 at 60 mph! Most irritating...

Almost despairing I finally checked my old standby, the Radio Shack catalog. Ah yes, some tiny rare earth button magnets (Cat #64-1895, about $1.50 for two). Maybe they would help, so I bought six. First try I put two on. They stick themselves to the "wings" of the rotor (I don't know what the wings are for, maybe to adjust the damping of the movement). Wow, now I had 120 mph indicated at 60!! They are really strong!

Taking one magnet off, I was back in the right ballpark, and from then on it was just a matter of trial and error to home in on the right calibration. Installing the magnet at a smaller radius reduces the indication sensitivity, and vice versa. The magnets are so small that the out of balance isn't a significant factor.

Now I don't have to go through those "How many revs/per 10 mph?" calculations to check my speed any more. If it says 60-then 60 it is!

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