The maintenance costs per mile were modeled using the HDM-III model. The more updated HDM-4 was considered, however the new updates only give the user room to account for varying road conditions, and the TCO calculator is assuming unknown road conditions. The model breaks maintenance costs down into two sections, Cost of Parts and Cost of Labor. Cost of Parts is defined as cost of parts multiplied times “rate of parts decay”. Rate of decay is calculated using parameters from the model that are based on a given vehicle’s specifications. Cost of Labor is broken down into two parts: price of labor, and labor hours per mile. The price of labor is assumes 40 hours worked per week, and the number of labor hours is calculated based on vehicle parameters and the previously calculated decay of parts.
This model was developed in Brazil in the 1980s, so various economic and human factors needed to be updated when adapting the model for the current day conditions in the USA. Examples of this include higher cost of vehicles, higher quantity of skilled mechanics, better road conditions, etc. To take these factors into account, the model was calibrated by diving the labor hours by 12.75. This number was obtained by making the model fit the cost results of 10,001-14,001 pound truck, results which had been obtained from an OEM that the creators consulted.
The maintenance costs for alternative fuel engines were derived using ratios of efficiency between each alternative fuel vehicle type and a similar gasoline-fueled vehicle. After the previously described method of calculation was utilized to find the gasoline maintenance costs, the corresponding costs for different types of engines were estimated by using these efficiency ratios. It is important to note that the model yielded results that did not fit the trend for several of the shuttle bus and school bus options. This is likely due to the lower MSRP cost of shuttle and school buses, comparatively. In order to fit the model, these estimates were projected based on the trend resulting from the model in other fuel-type and vehicle type combinations. More long-term research on maintenance costs for alternative fuel vehicles is required in order to improve the preciseness of average maintenance cost estimates.
As with pre-set purchase prices, pre-set fuel economy estimates, and pre-set fuel prices, all users have the option to either rely on pre-set fuel maintenance costs or override those pre-sets to customize and improve the precision of TCO estimates.