At first I just had a suspicion that the speedometer was a little off. That made me perform some tests and just as you said I used the marks on the highway that the police helicopters use to determine the speed of vehicles. In sweden the marks are set every 0,5km. I set the tempometer at 100km/h and did several readings, all pointing to 20,8 sek. According to my calculations that would be 86,5 km/h so it's almost 15% off. (However, how I see it, the first 5% is standard deviation that most cars have). Thank you guys for looking into this problem.