In April last year I bought a 2008 Ford Focus. The tyres were relatively new, all apart from the front right which was more worn than the rest. I've driven the car roughly 7,000 miles in nearly 11 months.
I measured the tread depth on each of the tyres and 3 of them have around 4.5mm of tread left but the front right has only about 2mm left (legal limit being 1.6mm). The worn tyre is a different brand to the others which suggests to me the previous owner replaced just that one (maybe after an unrepairable flat or something?).
My question is: Would it be alright to change just the worn tyre for a new one with the intention of swapping the others to the same tyre make (Pirelli P7) once they need it? Would it be better to swap both front ones and just accept I will lose 4.5mm of good tread? (The spare is a space saver).
I've found a couple of other questions on this subject but none that seem to discuss changing a tyre that is much more worn than its opposite tyre when the other tyre in the pair still has reasonable life in them.
As far as I understand there could be differences in the characteristics of tyres so that in heavy breaking it might behave differently. I also read there might be insurance implications due to 'improper maintenance'.
I'm not so concerned about the cost of this (if they need replacing, they need replacing...) but I don't necessarily want to throw away a part worn tyre if I don't have to.
Update: I'm specifically interested in how this might affect the handling and if the effect would be adverse.
Outcome: Based on the comments here I actually ended up changing both front tyres for new Pirelli P7s and moving the less worn tyre from the front to the back where there was evidence of cracking in the side-wall of one of the rear tyres. I disposed of a cracked side-walled tyre that had roughly the same 4mm wear on it as the front one I'd swapped. I guess I lost out a little bit but really the equal front grip is probably worth it in the end!