My approach has been to drive it until its reliability is in question or some required repair is big relative to its residual value.
Discussion
Fair point, I've got a 97 sierra that worked great till the transmission went out over 3rd gear. I'll say its not on the road anymore but its the best tractor I've ever had. Pulls stumps like a champ
Hey I just recently got rid of my 97 sierra! Loved that truck. Only reason I let it go was because the frame was starting to rot and the dealer offered me 1000$ for it (scrap value)
Mine stopped shifting into 3rd and reverse, we rebuilt that transmission 5 times until we learned that particular model couldn't be rebuilt. You had to have a factory new to replace it. But hey we got 386k miles out of it first
also don't forget srcurity: being in a car or another when a drunked guy hits you could make the difference from life and death. Still Toyota rav4 from early 2000 is the car I would put my life on