I would make sure the ones on the back are same size.

If different sizes they will overwork the ring and pinion. I had a friend who fried his diff that way. It got hot and lost its lubricity and smoked the bearings. l I guess if you have awd you might want to just buy the one new tire so they all match.

And use the oddball as a spare?

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.