I use the mile markers while traveling a sufficiently open stretch of road.
I like to do several 1-mile sections for an estimated average.

To figure out how many seconds per mile you SHOULD be moving at a given
speed:  60 * ( 60 /Indicated speed) = seconds per mile.

To figure out how fast you're ACTUALLY moving given a certain number of
seconds per mile:  60 / (Seconds in a mile / 60).

So, if you're traveling at 51 seconds per mile, 51/60=.85, and 60/.85=70.6
mph.

Hope that helps to put you in the ballpark.

I've said this dozens of time, but I'll say again that both my '85 Westy and
'84 Westy have fairly accurate speedometers with stock tires, which I
understand is a bit unusual.

2009/10/12 Melissa Mourkas <westywoman@mac.com>

> I have come to the conclusion that I am not driving as fast as my
> speedometer indicates as people are racing around me and my speedo
> says I am doing 70 mph (and I thought this 1.9L was just really
> peppy!).  I have 15/65/205 tires/wheels.  I have been unable to
> conduct a test using mile markers as they are nearly non-existent
> around here and, no, I do not own a GPS.  (I like maps). My shift
> points seem about 5 mph off the marks on the speedometer as well. I
> followed a thread on the Samba regarding this disparity:
>
> http://www.thesamba.com/vw/forum/viewtopic.php?t=345224
>
> They were claiming as much as 10 mph under at 70 mph (actually
> traveling at 60 mph).  Has anyone on this list experienced this and
> have you tried this ratio-adjusted speedo cable shown in that thread?
>
> The vendor for the adjusted cable is:
>
> http://www.tacomaspeedometer.com/index/index.php?
> option=com_content&task=view&id=15&Itemid=28
>
> I apologize if this has been covered ad nauseum in the past.  I will
> try to come up with something more original next time.  In the
> meantime, thanks to all who choose to reply.
>
>
> Melissa Mourkas
> '84 Westy Honey Bee
> Sacramento, CA
>

--
Chris S.
Disclaimer: "Death and serious injury may occur"