John Wakefield talks about using 3600, a stop watch and a calculator.

I do something like that but I use a Robic sports watch.  This watch does
lap and average m/kph readouts.  You enter in the distance and press
start/stop every mile/lap and it gives your speed.  I have used method to
fine out speedometer error.  My GTI and rabbit truck are both 5 mph slow so
I have to add 5 to my indicated speed to get my actual speed.  This is between
40 to 70 mph.  My Golf is 3 mph fast at this speed range so I have to subtract
3 from the indicated speed.  I have not done this yet to the vanagon so I
don't know its accuracy yet.

I also check the odometer error too.  I drive 120 miles/day and part of
my commute is a 31 mile stretch of interstate.  I use the 30 mile markers
as a reference.  For example, in my 92 Golf.  I found that driving 30 miles
my odometer will read 30.8. which gives a 2.67% error.  This is good to find 
out what your exact mileage is.  I gave been doing this for several years. 
It is useful especially after getting new tires because they will read
differently than old ones and if you change brands or sizes.

BTW, the sports watch I mentioned costs about $35-40.  There is a more
expensive model too.  You can get them at most mail order racing supply houses.

Jonathan
jdove@gsvms2.cc.gasou.edu