You can certainly mix tires as long as you keep the same tires and approximate tread depth on an axle. One of my Boxsters was delivered from the factory with 2 different treads (Pirelli) designed for each axle and N-rated too. The strict German tire requirements due to their highway speeds allow different tires as long as the same tires are on an axle. Once had a blowout and had to negotiate a replacement with a rental company and the tire store in Germany and the tire store was required by German law to replace two even if only one went bad and the rental outfit had to pay for two.
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