Billy,Reliability may be a major factor in your analysis.Every month you can run an old vehicle you are AVOIDING the cost of purchasing new and incurring steep depreciation costs, which are usually the largest cost factor by far. Since your average monthly costs for the life of the vehicles are probably already very low vs the cost of replacement, any financial comparison will likely favor continuing to run existing vehicles. However, as you run any equipment to the limits of its practical useful life, understanding their reliability becomes increasingly important. Once a vehicle becomes unreliable, it loses its utility rapidly, and total costs (vehicle + consequential + opportunity) rise sharply.I have seen capital equipment evaluated using reliability curves to estimate useful life, MTBF, and other related metrics. Total costs might include downtime, inconvenience, disrupted processes, lost time of personnel, safety risks, and lost opportunities. Whenever these non-vehicle total costs start kicking in at unacceptable levels, then replacement is usually overwhelmingly rational. This approach might give you what you need to devise a replacement plan.
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