wantone:

I've driven in the US and my average speed was 65. No cameras, true, but there are state troopers everywhere. I don't see how and do not know anybody who can maintain high speeds (>120mph) for more than a few seconds.

You have not driven here all your life and I doubt you have crossed the wide open plains. The answer is a valentine one and good reflexes. I have averaged 86 mph to Montreal over 18 hours. I doubt you could find enough open autobahn for that distance and average. It takes a lot of speed to maintain that type of average. 

Merry Christmas.