The 777 Series Veeder-Root Mechanical Hubodometer is mechanical and has rugged, reliable operation for all applications. The hermetically sealed case is filled with inert gas and a double O-ring protects the unit in any environment. The proper ratio must be selected based on the rolling circumference of the tire that is mounted on the same axle on which the unit is mounted. A guide to the ratio for most available tire sizes is available. Built in a die-cast zinc case.