IF a German 1M owner would scan their fahrzeugschein
(car registration paper)... ???
...in one of the boxes the German TÜV tests a cars true top speed/V-max(averaged in both directions on a flat road). What ever the number is... I have found that most cars can do 10 to 20 kph faster (that that number, on the speedo!) on the road going down a hill/autobahn!
See box #6
for this car's V-max... 230 kph in this case. So the car's speedo would show about 240-ish while the car's real gps speed is 230 kph.