I worked up a spreadsheet a few years ago that tries to summarize cost of operating our vehicles.
In it goes gas, oil, tires, estimated repairs, and estimated depreciation - repairs are guessed at from history and sheer hand-waving and assume 10,000 miles of operation, and depreciation figured from blue book differences on the cars as they are and +10k miles. The depreciation doesn't count advancing age, because we're keeping the cars, so whether or not the car is driven, it's going to age; I just need to know how much more miles affects the resale value.
It used to be that because of the lower cost of diesel (vs gas) and the almost zero depreciation that use of my truck (20 years old and about 200k miles, but never breaks) incurs, its cost of operation was similar to that of a much newer car that gets twice as many miles to the gallon. However, diesel in CA-USA now costs easily 35-50 cents per gallon more than regular, and all of the cars are older, so they're not depreciating as much either (a car with 110k miles isn't worth that much more than the same age car with 120k miles, so that 10k miles doesn't cost as much as putting 10k miles on a brand new car).
It's useful to have an idea of actual cost per mile for car use... My son was considering driving to SoCal for a friend's wedding because of the "high cost of flight" until I told him his car cost 25 cents per mile, both ways - suddenly, the air travel cost about the same and was much less tiring for a short weekend trip (500 mi each way). Also gets me out of driving my wife to the airport 70 miles away for short trips - the extra trip each way in the car costs the same as parking for two days.
