There is a simple formula for this. When the adjusted value of the vehicle is less than the cost of repairs to keep it road legal (derivable). Adjusted value is found by starting with resale value and weight it up or down based on how you feel about the car. I love this car = add 50% to resale, but if you think “Kill me now” every time you get in, reduce resale value by 50%. The assumption is you’ve already stopped doing ‘recommended service’ by this point, so if the repairs required to keep you road legal and safe exceed the adjusted value, dump it and get something else.