You know what?..That is a good question...I'm banking I don't have a total tire failure (my glass is half full)..Keep them babies pumped to 40psi to reduce heat and friction wear for one thing..But knowing how I am..I just might invest in 4 new tires if the other three have less than half life left..Otherwise, I would put the spare and a new tire on the road on the same axle and put a used one on the spare mount...that way at least the tire wear patteren on that axle is symetrical and then start rotating front to back only...in any case, I'll deal with that situation in the best way, as it presents itself...