odometer (ōdŏm´Ĭtər), instrument provided in an automotive vehicle to indicate the total number of miles that have been traveled. The odometer generally shares a housing with the vehicle's speedometer and is driven by a cable that the two share. When the vehicle is in motion, this cable moves a series of gears in the odometer, turning a set of numbered drums that count the miles traveled. Some odometers, called trip meters, are manually resettable to zero; these are used to measure the lengths of individual trips. Odometers are often equipped with means for indicating if they have been tampered with, i.e., set back to make it appear that a vehicle has traveled fewer miles than it actually has. An instrument similar in design and function to the automotive odometer may be fitted to a bicycle.