Effect on accuracy with US speedometer in Euro model?

I recently bought a 1980 500se that at some point was fitted with the instrument cluster from a US version of a W126. It seems to me that it is off but I'm not sure by how much. Does anyone here know a formula for
figuring the actual speed? I know from the owners manual for the car the top speed shifting points are different in the 500se than on the speedometer but that is little use. Thanks for any and all help.
John
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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
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Yes, Frank, it isn't just a German Federal Road Ministry invention...
:-) DAS
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Do they obey the 500 meter increment all over europe?
Frank
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Obviously not. For a start we don't know what a kilometer is on a British road... :-)
The broader point is having distance markers along the motorways, is it not?
DAS
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If the speedo is found to be off, is there a way to recalibrate it through the onboard computer systems?
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