What's your top speed?

car drivers will always overclock your speed... most cars are calibrated about 5mph slower on their speedo to keep people within speed limits... if a car clocks you at 30mph chances are your going about 25 or 26 in reality.

It is usually a % thing rather than a set number. Real speed 70 - speedo reads 77, Real speed 30 - speedo reads 33.
Best to use sat nav for real speeds (you can then go a bit faster than you thought!)