Hi Fabo,
I constantly monitor mine because i too am unsure at times, and curious.
However, i can not find any discernable speedo errror to the point where i believe it is actually correct.
The only way to really tell would be the measured/timed run on the freeway (5Kms traveled at a constant 100 Km/h takes exactly 3 mins) for the speedo to be correct.
I know my SFS is correct. Don't know about the 1199 yet?
Regards,
Brad