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Buy tires and keep car for 12-36 months, or replace car now? 
New tires will increase the resale value of the car; while not by the full cost of the tires, it will not be entirely a sunk cost. You'd need to factor that in and find out how much the new tires increase the resale value of the car to determine how much they would truly cost you.  However, I suspect they would cost you less than a $25,000 car a year early would.  That new car would cost some amount over time - it sounds like you buy a new car every 8 years or so? So it would cost you $25/8 = $3.3k/year.  That would, then, be the overall cost of the new car a year early - $3.3k (as it would mean one less year out of your old car, so assuming it was also $25k/8 year or similar, that year becomes lost and thus a cost).