Pretty much what the guys have said already.
a search will bring up results on this.
basically it depends on your riding style.
factors that affect mis matched tyres:
If you ride very hard you may encounter some strange handling
tyres are designed for the tread to line up in the wet, mismatched tyres are not as good as the designed pair in the wet.
Profile also affects handling and grip, I would if I had to have an unmatched tyre stick to the same profile ie. round or pointed.
Also as Steven said ensure compound is either the same or front softer than rear. Sprints push the front more than a lot of bikes which is why cupping is an issue with the Slipstone BT 020.
If you just commute and do not attack the corners with great vigour and the tyres are similar profile it most likely will not matter much for a short time.
Personally I replace a set, but I often need to replace both at once anyway (not very normal)

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DaveM
