National Odometer Day is May 12. An odometer is an instrument that indicates the distance traveled by a vehicle. It may be electronic, mechanical or a combination of both. Odometer comes from the Greek words hodos meaning path or gateway and metron, meaning measure. In some countries, an odometer is called a mileometer, milometer or tripometer. Odometers were first developed in the 1600s for wagons and other horse drawn vehicles to measure distances traveled. 90,000 miles showing on this car odometer.