speedo accuracy

[r.c.brown@ieee.org (Roger Brown)]

> > If I thought about it I'm sure I could figure out why speedos
> > are less accurate at higher speeds mathematically but I'm too lazy.
> 
> its due to driven wheel slip % at higher speeds.
> 
> > At first I thought it was the flexibility of the cable making it unable to
> keep
> > up with the speed of the vehicle but that energy has to go somewhere and
> > eventually it would just twist off everytime you drive
> 
> answered yourself on that one.
> 
> > Probably has to do with the higher tension
> > of the spring on the needle at higher speeds!  HTH
> 
> nope, spring is linear, the tension goes up even from 10-20 mph. going from
> 10-20 adds the same tension as going from 100-110.


Actually if you tear apart a speedo and see how it works, its quite a simple system.
It uses only 3 parts, a spinning magnet inside a lightweight aluminum cup to which is attached a coiled spring (and the speedo
needle).  Magnet spins, induces current in cup which in turn produces a magnetic field which tries to "follow" the spinning
magnet.  There is no friction or contact involved, the only thing that can "age or wear" is the magnet that spins.

More info:
	http://www.alfaclub.org/techstff/speed.htm

There is variation between speeds for sure, the one in my '82 read ~5% slow, I replaced it with an '84 GTI speedo and it read
almost dead on.  Both were the same calibration number.