In the old days there used to be marked speedometer test sections on the interstates (maybe still are?).  Ten mile marked section you could use to check the odo, and if you were able to drive at a constant observed 60 mph you could calculate your speed error with a timer.  My '84 was only about 5% high using this technique, including the odometer.

Don't try using mileposts on anything but the interstates though.  Often secondary roads have been realigned over the years and mileage markers are simply removed, or changed distances not re-posted.  You may go from milepost 45 to mile post 47 in a mile, or milepost 45 to mile post 46 in two miles!

Stuart

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Mike S
Sent: Tuesday, May 07, 2013 8:57 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Fwd: Re: Re: speedometer accuracy

On 5/7/2013 11:01 AM, David Bjorkman wrote:
> 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.  There should be a place you can
> check which satellites are being used by your GPS and the expected
> accuracy.  One may be surprised to find that that error can be more than
> just a few feet.

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/