Sometimes it works well, sometimes it's a bit of a disaster.
My own experience is that some brands mix better than others. For instance, tyres like Pirelli's SC-1 and SC-2 are so similar in shape to Metzeler's Racetec K2's that they might as well be the same. I've run a mix of these at the track with good results.
However, Dunlop and Michelin tend to be quite different in shape to Pirellis - just as a rule of thumb - and a mix can feel weird as the tyres 'roll in' differently.
It's not foolproof advice, but I have to say compare the shape of your tyres before committing. I.E, some are broad, others are more 'peaked' and tall in their shape/profile. If you can get tyres that look similar its a good start - for instance, I've heard that Pirellis and Metzelers use the same mold shape and even come from the same factory.
Of course none of this takes into account carcass construction, belt type or sidewall stiffness.
But as someone else here has mentioned...if you're riding 7/10ths on the road you will probably be fine regardless.
O.B
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