speedo accuracy

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

> >BUT when I pass one of those portable police radar "Your Speed Is..."
> >trailers, the speed indicated by the radar is nearly 10% higher than my
> >speedo and tach/theoretical speed chart would indicate.
> >
> >I have to assume my calculations AND my speedo are off, or the radar
> >trailer is off...WTH?
> 
> I pass one of those every day and the speed indicated is always 3 mph less 
> than what my speedo indicates.  I've got GTi 14" wheels with 195/60's.  
> Also, my last speeding ticket said 62 in a 35 mph zone.  When I passed the 
> cop, my speedo indicated about 67.  Nearly lost my license that night, 
> almost 11 years ago.

I think some of those "Your speed is" radars are set to read high to make you slow down, while others I've seen are quite
accurate.  In northern California on I-5, they have some tight curves for the truckers and have added radar speed readout signs
and I've found they are dead on compared to my GPS speed reading.