As per bloody_knuckes comment, there is a rational method based on cost, that will determine the best point to get rid of a car. Businesses use a similar appraoch; if the typical annual cost of ownership per year starts going UP, then is a good time to trade. With businessses there is tax and depreciation allowances to consider of course.
The offical business rule is that "if the incremental (current) annual ownership cost starts to EXCEED the CUMULATIVE average annual cost to date, you trade. Any individual can do this, but you must keep track of all your costs and know what the depreciation is in each year of ownership.
My retired accountant sister practices basically wearing out the car until it either becomes unsafe, or there will be a repair bill that exceeds the remaining value of the car. That usually happens when the car is about 15 years old.
Personally, we keep cars until they become unsightly with corrosion, they become unsafe, or they cease to be “repairable” due to parts unavailability or excessive cost.
Some past cars and their reason for disposal:
- Dodge Dart 1965, chassis rusted out, car became dangerous, could no longer be jacked up! Scrapped for $30.
- Dodge Colt, 1977; very hard to start, needed new choke and other engine parts no longer available. Scrapped for $60 in 1997.
- Chevelle Malibu, 1966; crashed in 1977 at 98,000 miles, car still in excellent condition before crash. Scrapped for $100.
- Mercury Comet, 1971; given to mother-in-law at 50,000 miles in 1978; new job required better car.
- Ford Granada, 1976, drivability problems and corrosion, sold at 108,000 miles in 1989 for $750.
- Chev Impala, 1984; given to son for college at 200,000 miles in 1996, in near perfect condition. Son drove it another 8 years, then sold it for $700. Kid who bought it is still driving it.
- Chev Caprice, 1988; new job required newer car; sold in 2007 in near perfect condition for $1400.
P.S. Yes, we “get rid of” cars since we don’t have room for non-functional or redundant objects that size. A fellow down the street kept a 1970s Volvo (inherited derelict) and a pristine 1976 VW Rabbit (inherited from father) in addition to the late model Corolla family car.
He’s moving this summer and the neigborhood will breath easier with the Volvo gone from the driveway.