Unknown to many drivers, a number of GPS units store your "maximum speed" since you last reset the device's maximum speed log.
For example, the Garmin Nuvi 1450 stores it in your route information data (you may have to scroll down to see it). If you have never reset it, it will show the maximum speed you ever traveled (with the GPS turned on) since you began to use the device.
You can also use it to spy on other drivers of your car (providing they turn on the GPS when using your vehicle). So if a heavy-footed teen or friend wants to use your car, you can track their max speed after they return it, if they had the GPS turned on and don't know enough to reset it.