Operating cost records of 248 automobiles operated by rural mail carriers in Iowa, 43 in Indiana, and 2 in Alabama, covering 170 routes, were analyzed to determine the operating cost differentials for pavement, gravel, and earth road surfaces. Operation covered the period from November, 1935 to January, 1937. An analysis was made in 1937 of the records of 159 cars and supplemented the following year with a group of 134 additional cars. The close agreement in the results for the 150 originally studies and the 293 cars which comprise the final total, indicates the reliability of results when using what might seem to be a relatively small number of cars. However, the results obtained apply directly only to cars operating under conditions similar to those encountered by rural mail carriers. The detailed daily records of these cars covered such phases of operation as miles of travel on each type of road surface, rates of travel, weather, number of stops, load, amounts of gasoline and oil used, and expenses incurred for tires, maintenance, garage, license, taxes, insurance, depreciation, interest, and extra help. Records submitted were analyzed to determine the average total cost of operation on pavement, untreated gravel, and earth roads for a complete year and for the four seasons. The effects of annual travel, total car mileage, and vehicle age on costs were studied, as were the effects of type of road surface on rate of travel. Segregated costs of gasoline, oil, tires, and maintenance were obtained and compared on the basis of type of roadway and season of year. An application of these data was made to determine the traffic volume necessary to justify changing from earth to a gravel surface on a typical Iowa county road, and to justify the cost of snow removal.