[demick]

I think you need to subtract out what you think you could sell the car for today. Otherwise your calculations do not make any sense, as your cost will always start out sky-high, and drop over time.
Example: purchase used car for $20K, drive it 1K mile in the first month. Cost per mile is $20 per mile. Continue to drive it 1K per month, and after a year your cost is now $1.66 per mile. That's a drastic difference when in reality, your cost per mile didn't really change much over that year.