National Odometer Day May 12th provides an opportunity to learn a little bit about the odometer. An odometer is the instrument that indicates the distance traveled by a vehicle. It may be electronic, mechanical or a combination of both. The word odometer comes from the Greek words hodos, meaning path or gateway, and metron meaning measure. In some countries, an odometer is called a mileometer, kilometer or tripometer. Vehicle with 89,999 miles on the odometer, ready to turn over to 90,000 miles..