jel<jelow1966@yahoo.com> haute in die Tasten:

The most convenient way to find out your actual groundspeed is using a GPS
Handheld receiver for that. They start at 200 Euros, maybe you can borrow
one or get it used on ebay. I can recommend my Garmin GPS 12.

If you happen to be in Germany, you can also find out the speed by counting
the seconds with a stop watch at the autobahn. Get yourself a straight
piece of Autobahn with little traffic (try it on sunday morning), travel at
constant speed (cruise control) and count the side marker poles. They are
installed every 500 metrers (quite accurately set), so two of them give one
Kilometer. If you need 60 seconds for one Kilometer, you travel at 60 kph,
30 Seconds are 120 kph, 20 seconds are 180 kph. Note the reading of your
speedometer and the time for one kilometer, the rest is five minutes work
and a pocket calculator. Some chronometer wrist watches even feature a
speedometer scale, called tachymeter. Simply stop the time for one
kilometer and read the speed from the tachymeter scale. The existence of
accurately placed side marker poles is essential for this method, but I do
not know if they are also available in other countries.

BTW: It may be a good idea to leave the metering and noting to a co-pilot
and concentrate on smooth and constant driving. On an older Mercedes a
speedometer which reads 10% too fast is not an unusual thing.

Frank

Frank

If the speedo is found to be off, is there a way to recalibrate it through
the onboard computer systems?