[bumby]

>salvage the old car (puts some money in your pocket) and replace it (with a similar used one if necessary) for less* than the cost of the repairs to the old one.*

I mean, sure, if that's an option. But there's always risk in swapping out one used car for another. If I can only swing $2.5k, I'd rather put it in the vehicle that I've had for 10 years and know the maintenance background on that roll the dice with, as you say, another "old heap."

It's not about being sentimental or irrational. If you're in the situation where you have to weigh these decisions, you tend to have limited options. It's not likely one of them is "trade in my old heap for something great and reliable." In these situations, taking money that could go to your car and instead go out to dinner would be the irrational choice.

>Why spend $6K on repairs if you can buy a similar working car for only $5K? Why make your net "position" $1K worse with flawed logic?

I don't think it's flawed. I can reduce the uncertainty on my utility a lot more with $6k on my own vehicle than I can on a $5k random car. When you get to that low of a price, most cars are going to have maintenance issues. There's nothing guaranteeing that I spend $5k on something else to only have drivetrain (or other major) issues six months down the road.