Mail was a haphazard affair in colonial America. Letters between cities were carried by whoever was available, and post offices were little more than sacks of mail stashed in the back room of your local tavern. Many colonists would make up to 5 copies of a letter and send them in five different directions just to make sure one of them made it to its destination [source: PBS].
In the 1760s, the British government tapped Franklin to make some sense of the colonies' slapdash postal system. A man of letters himself, Franklin dove into the task with a firm resolve to speed up communication between the colonies.
He started by touring America's major postal centers, studying ways to standardize streamline mail delivery. Along the way, Franklin charted the distances between postal stations by attaching a geared device to the rear wheel of his horse carriage. Every 400 revolutions made by his carriage wheel would cause the device to click ahead one mile (1.6 kilometers). By the end of Franklin's tour, he had gathered a stunningly accurate survey of early colonial roads.
It wasn't the world's first odometer; rudimentary mileage recorders had been appearing as far back as ancient Roman times. Franklin's design also wasn't the last odometer; inventors in Nova Scotia and the Midwest would independently conceive of similar devices in decades to come. However, none would put the odometer to such practical use as Franklin.
Most modern automobile odometers are electronic, but you can still see a slightly worn version of Franklin's odometer at Pennsylvania's Phillips Museum of Art [source: Ben Franklin Tercentenary].