I haven't tried it in the X5, but in my other cars you could use the trip computer's average speed reading to see current actual speed. Basically get up to a steady speed and check your speedo.

Reset the trip computer and the average speed display should now be your current actual speed. That was always 2-3mph lower than the speedo and seemed more accurate.

Can't say that it's something I really worry about. I try not to exceed 10mph over the posted limit when I want to be law-abiding, and when I pass a cop I'm happy to know that I have a little bit of extra leeway from the optimistic speedometer.