Back before the days of speed monitor displays that tell you what your speed is as you drive by like this, I'd have a friend drive at a set speed then see what my speedo said. If he was driving 60 and my speedo said 55 I'd just keep in mind that my speedo reads 5 MPH slow. These days I can use the speed monitor to compare actual speed to displayed speed.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.