I thought my speedo was pretty accurate. I would drive along those odometer test counters and look down at my odometer and it would be dead-on. So I naturally assumed that the speed indicated on the dash would also be accurate.
Well as a retirement present, the company gave me a Tomtom 250 XL GPS. I placed this the window. I noticed it would also read the speed that I was traveling at and it would go red if I was driving too fast. It was then that I glanced at the actual speed and noticed that my indicated speed was even faster.
So while my speedo was telling me I was going 110kph, the GPS was telling me I was going only 105kph. I checked this about a dozen times and the difference would always be 5kph regardless of speed.