Mechanical odometers have been counting the miles for centuries. Although they are a dying breed, they are incredibly cool because they are so simple! A mechanical odometer is nothing more than a gear train with an incredible gear ratio.

The odometer we took apart for this article (pictured above) has a 1690:1 gear reduction! That means the input shaft of this odometer has to spin 1,690 times before the odometer will register 1 mile.

Odometers like this are being replaced by digital odometers that provide more features and cost less, but they aren't nearly as cool. In this edition of HowStuffWorks, we'll take a look inside a mechanical odometer, and then we'll talk about how digital odometers work.