> Before you go blaming the speedo (which is well documented as being
> off) and take the GPS as the final word. do not forget that there is an
> error in accuracy in your GPS also. 
"Modern GPS units don't compute speed based on how long it takes to move
between points, so are not subject to errors in location.
They measure the Doppler shifts of the carriers."
"Generally most manufacturers claim a 0.1MPH tolerance with speed
readings, however will go as far to say a 0.5MPH inaccuracy is possible,
to make allowances for any satellite signal variations beyond their
control." - http://gpssystems.net/accurate-gpsbased-speedometer/"

Mine is spot-on. I used my Garmin 400T to set my Scan Gauge and
it has proven accurate to within 1mph in all situations, at all speeds,
from neighborhood speed control signs (yes, I know they CAN be inaccurate)
to highway SCS.

My speedo has never been corrected since installing larger rims/tires so
the error is 7mph over at 70mph (OE speedo indicated). I just use my Scan Gauge,
which sits on the steering column plastic cover, just under the inst. cluster, and
get lots of information besides speed from it. 

Bob