mile is likely to be lower than if the 50,000 miles is spread over five 

 years. This point requires some modification of the relationship of re- 

 pair costs to total miles travelled when annual mileage travelled varies. 



In these analyses it was assumed that vehicles would be traded prior 

 to major overhauling or engine replacement. Aormal mileage per year 

 was assumed to be 10,000. Based upon data from assembly firms, dollar 

 costs per successive 10,000-mile intervals were constructed for each truck 

 size. The accumulation of these through the 10,000-mile interval preced- 

 ing major overhauling or engine replacement yielded the "cycle cost." 

 Regardless of age or mileage, some "maintenance" work is required, 

 providing an irreducible base to which "variable repairs" are added. 

 Maintenance is estimated at 1 percent of new cost per year. 



The following formula was used to calculate repairs and maintenance: 



RM = 'C + (Y-X.01 N> ^, 

 m 



In the preceding formula: 



RM := annual $ cost of repairs and maintenance for specified 



annual mileage m'. 

 C = "cycle cost" — based on increasing rate per mile for 



period prior to major overhaul. 

 Y ^ number of years to major overhaul at m' miles per year. 

 X = number of years to major overhaul at 10,000 miles per 



year. 

 N = new cost of vehicle in dollars (truck -)- ^ body or 



trailer, if separate) . 

 m =: miles in cycle to major overhaul, 

 m' = specified annual mileage. 



Oil. Costs per mile decrease with annual miles travelled. Although 

 oil consumption increases with aggregate mileage, calculations in these 

 analyses are centered on average rates of consumption for the life cycle 

 of the engine. Furthermore, proper servicing requires oil changes based 

 on time periods when annual mileage is low. Hence, it is assumed oil 

 will be changed every 2,000 miles or 90 days, whichever results in most 

 frequent changes. Oil consumption for periods between changes is esti- 

 mated at one quart per 250 miles for vehicles of 5-quart crankcase ca- 

 pacity, with consumption for vehicles of different capacity proportional 

 to this rate. A price of 30 cents per quart was used. 



Lubrication. Costs per mile decrease with annual miles travelled to 

 the minimum level and are constant beyond this point. It was assumed 

 vehicles would be serviced every 2,000 miles or 90 days, whichever 

 occurred most frequently. Costs per job were increased with truck size. 



Tires. As truck size increases, the size and ply of tires increases. 

 Hence, the cost of a new tire or of a recapping rises. However, as size 

 and ply increase, the original mileage obtainable, the number of re- 

 capping possible, and the mileage per recapping increase. The number 

 of tires per vehicle increases as dual wheels become standard (or as a 

 tractor-trailer is used) . 



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