As others have said, the difference in diameter from 17,000km of driving will be minimal and I doubt it's going to cause any issues. If you're that concerned put it on the front left and get it scrubbed down quicker.
I would also ways do a 5 tyre rotation for a couple of reasons.
Firstly, when I change tyres I will more than likely change brands and try something different so I'd want them all to be the same.
Secondly, tyre rubber degrades even if it's not used and is past it's best by the time is 5-6 years old (starts to go hard, doesn't have as much grip, aquaplanes a lot more, starts to crack,......). This is more noticeable on some tyres, less on others. I had MT ATZ 4-Rib on my JK and after 4 years the rubber hardened up and they were dangerous in the wet. They still had over 50% tread depth left, with some siping still visible but if the roads we wet I could not go around a roundabout regardless of how careful I was without the back end stepping out. The degradation will be even worse if the spare is on the tailgate (like on the JK) with no cover and in full sun all of the time. Unless you do lots of km per year, the chances are you'll only need new tyres once every 4 years or possibly longer, and there's a good chance your tyres were a few months (or even years) old by the time they were fitted to your car. That means that even though it's not been used, it could already need replacing anyway by the time your 4 road tyres need replacing. Might as well include it in your tyre rotation and get some use out of it.
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Silver MY10 JKU CRD Auto - SOLD - Now just loitering for the hell of it
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