Mixing tire sizes is a short topic because the safe answer is almost always no. Same axle gets the same size, load range and tread. All-wheel drive wants the same circumference at all four corners, not just per axle. The exceptions are specific and stamped that way from the factory.
Same axle
Left and right on one axle share a differential. Different diameters make that differential work as if you are turning all the time. That is how you wear a carrier. Different treads on the same axle are how you split traction in a panic stop. Buy a pair. Better, buy four.
AWD and 4WD
Full-time AWD (and many 4WD systems in 4Hi on pavement) couples the axles. A few tenths of an inch of circumference difference is enough to wind the transfer case. Manufacturers publish a maximum wear difference, often 2/32 to 4/32 of tread. That is a wear spec, not permission to run a 265 in front and a 275 in back.
The exceptions that are real
- Staggered OEM setups. Some sports cars ship with a wider rear tire. That is a factory pair, not a license to invent your own stagger.
- Temporary spares. A spacesaver is a different size on purpose. Drive slowly, and only to the shop.
- Dually rears. Dual-rear-wheel trucks run two tires per side in the same size. That is not mixing. That is two of the same.
If you are replacing two, put the new pair on the rear (on a two-wheel-drive car) unless the manufacturer says otherwise. If you are upsizing, upsize all four and check the diameter on the comparison against the door-jamb size.
275/65R20 and 285/60R20 can land near the same height and still be a mix: different width, different contact patch, different load construction. Do not treat the diameter chart as a mixing chart.