A trail roadometer designed and used by William Clayton, 1847.

William Clayton was among the first pioneer group to reach the Salt Lake Valley. In May 1847 he designed, and Appleton Harmon built, this wagon odometer to measure road distances along what would become the Mormon trail. Clayton found that his wagon wheel turned 360 times per mile. The carved, wooden, gear-driven counter, mounted on the wagon, registered complete revolutions of his wheel to measure accurate mileage between campsites, landmarks, and river crossings for the benefit of travelers following after. Clayton used those distances to compile his “Emigrants’ Guide.”