According to Dictionary.com, a speedometer is an "instrument that indicates the speed of a vehicle, usually combined with a device known as an odometer that records the distance traveled." Odometers date to as early as 15 B.C. in Rome. Automobiles manufactured before the 1990s use mechanical speedometers and odometers. The electronic speedometer and odometer give the manufacturer additional choices.
Speedometers and odometers work by counting the revolutions of the drive train of a vehicle. Manufacturers calibrate these devices to operate within a 1 to 5 percent tolerance of the indicated speed and mileage.
The history of installing speedometers on automobiles and states' adoption of speed limits are closely related. According to HowStuffWorks, some early automobiles had two speedometer dials. A smaller dial inside the vehicle allows the driver to know his speed, while a larger one on the exterior of the vehicle indicates the rate of travel for a police officer. Improvements in methods that allow officers to determine a vehicle's speed eliminated the need for a second dial.
Installing tires that are larger or smaller than the manufacturer recommends affects the accuracy of the devices. A tire with a larger circumference covers more distance per revolution than a smaller tire. This causes the device to provide a reading that is slower than the vehicle is actually traveling. Drivers of vehicles with such modifications should take them into consideration to avoid speeding citations.